opeophysical research papersopeophysical research papers no. 21 absorption coefficients of several...
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OPEOPHYSICAL RESEARCH PAPERS
No. 21
ABSORPTION COEFFICIENTS OF SEVERAL
ATMOSPHERIC GASES
K. WATANABE
MURRAY ZELIKOFF
EDWARD C. Y. INN
JUNE 1953
Geophysics Research Directorate
Air Force Cambridge Res,:zah Center
Cambridge, Massachusetts
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ABSTRACT
Absorption coefficients, k, of 02, 0,, CO, CO2, NO, N20, NM, HI20, 112, CH 4, and NH 3 were measure-d in
the spectral region from 1050 to 2500 A. Using a linear photoelectric detector and an absorption ce!; at t he
exit alit of a vacuum monochromator, the improved method eliminated systematic errorb. Several thou-
sand k-values were determined and a number of new bands and continue were found. Energy state6 and
dissociation limits are discussed. The ionization potentials of NO and NH 3 were found to be 9.23 cv and
10.12 ev, respectively, by measuring the long wavelength limit of the ionization continuum. ioyiizati.,ji
cross sections of NO in the region from 1080 to 1345 A were obtained. Some of the data NS ere applied to
the formation of the D layer.
FOREWORD
Today the sights of the Air Force are being focused toward higher and higher altitudes. Altitudeswhich appear fantastic today will become the operational altitudes in the next ten years. With these alti-
tudes come problems which are quite different from those with which we are accustomed at the conventional
altitudes of today. At altitudes of 50 miles or higher, we enter into an entirely new realm of the atmos-phere which many prominent scientists like to call the chemosphere. It is a region of intense ultraviolet
radiation from the sun, and because of this huge influx of energy, it is a region of chemical change anti radical
reactions. Constituents of our atmosphere, which we are accustomed to consider as stable near the earth'ssurface, become readily dissociated and recombine to form compounds with vastly different properties.
Some of these gases are deadly poisons, some of these have violent effects upon the combustion processes of
reciprocating and jet engines, and others are corrosive to metal and plastic components of an aircraft.
Although these gases are present in very dilute ec.ncentrations, it quite possible that supercharging will
increase the concentratio, to the danger point. Such problems seem somewhat remote today, but we must
realize that considerable time is required to acquire information for the future in regions of the atmosphere
which are presently accessible only to occasional rocket flights of very short duration. In order to study
these phenomena at short range, the more fruitful approach has been to synthesize these conditions in the
laboratory through the use of known mixtures of pure gases hat are irradiated by ultraviolet light of wave-
lengths comparable to those which exist in the chemosphere. Reactions caa then be studied under con-trolled conditions and the role of each constituent can be determined.
This paper represents the first step in such an approach to the chemistry and composition of the high
upper atmosphere. Absorption cross sections of atmospheric constituents in the vacuum ultraviolet is a
basic property which is the foundation of all gaseous photochemical reactions. Up to the present time the
state of knowledge of absorption of atmospheric gases in the vacuum ultraviolet has been spottN and, in.,_:'-r, cases, quite inaccurate. The work described in this paper represents the first successful determina-
tiorn of continuous values of absorption coefficients of atmospheric gases in the wavelength region down to
1050 A by the use of a photoelectric method. The work has been carefully done, and is of sufficient caliber
to justify extension of the present absorption coefficients in the International Critical Tables down to 1050 A.
At the present time the information is of primary interest to geophysicists, physicists and photochemists
interested in high altitude phenomena, but it is highly probable that in the not too distant future, it willrepresent one of the fundamental stepping stones upon which we shall build the Air Force of the future.
P. H. WYCKOFFChief, Atmospheric Physics Laboratory
CONTENTS
Section Page
A bstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction .................. ..................................... 13
2. Experimental ................. ..................................... 14
2.1. Light Source .................. .................................. i5
2.2. Detector and Its Calibration .............. ........................... 16
2.3. Monochromator ................ ................................. 18
2.4. Grating..................................... 18
2.5. Absorption Cell and Gas-Filling System ............ ...................... 20
2.6. Procedure ................ .................................... 21
3. Oxygen ................. ....................................... 22
3.1. Results for the Spectral Region From 1900 to 1750 A ................... 24
3.2. Results for ithe Spectral Region From 1750 to 1300 .................... 26
3.3. Results for the Spectral Region From 1300 to 1050A . . ................. 28
4. Ozone ................... ........................................ 29
4.1. Preparation of Ozone and Conditions of Absorption Measurement ..... ........... 30
4.2. Determination of Purity ..... ........................ . .... .......... 3
4.3. Results and ]Dieussions ................ ............................. 32
5. Nitric Oxide ............... ..................................... .. 35
5.1. Purification of NO ........... ................................ .. 36
CONTENTS (Continued)
Smon pap
5.2. Results for Band Systems ............. ............................. 37
5.3. Absorption Continua ............... ............................... 39
5.4. Photoionization Cross Section of NO ........ ........................ .. 41
6. Carbon Dioxide .................. .................................... 42
6.1. Results for the Spectral Region From 1400 to 1800 A................. .43
6.2. Results for the Spectral Region From 1200 to 1.100 A ................. .45
6.3. Results for the Spectral Region From 1050 to 1200 .................. 46
6.4. Discussion on Electronic Transitions ............ ........................ 47
7. Nitrous Oxide ................... .................................... 48
7.1. Results for Region E-F ............... .............................. 49
7.2. Results for Region D ................ .............................. 51
7.3. Results for Region C .......... ............................... ... 52
0 7.4. Results for Region B .......... ............................... ... 53
8. Nitrogen and Hydrogen ............ ................................ .. 54
8.1. Nitrogen ............... .................................... .. 54
8.2. Hydrogen .................. .................................... 55
9. Carbon Monoxide ............. ................................... .. 56
9.1. Results for the Spectral Region From 1170 to 1600 ................. .56
9.2. Results for the Spectral Region From 1060 to 1170 A................. .57
10. Ammonia ................. ....................................... 58
9
CONTENTS (Continued)
Secti Pae
10.1. Absorption Coeficients and Continua .......... ........................ ... 59
10.2. Band Systems ............ .................................. 60
10.3. Ionization Potential of NHs ......... ............................ 60
11. Water Vapor ............ ..................................... 61
11.1. Results for the Spectral Region From 1450 to 1800 A ................. .62
11.2. Resuits for the Spectral Regiou From 1250 to 1450 .................. 63
11.3. Results for the Spectral Region From 1050 to 1250 A ................. .63
12. Methane ............. ....................................... 64
12.1. Absorption Coefficients .......... .............................. 65
12.2. Long Wavelength Li.t of CH4 Absorption ....... ..................... 66
12.3. Continua and Bands ........... ............................... 66
13. An Application to the D Layer ......... ............................. 67
14. Acknowledgments . ........... .................................. 69
15. Appendix Tables of Absorption Coefficients ......... ....................... 70
16. Rek ...ces ............ ..................................... 76
11
ILLUSTRATIONS
Fiure Pap
I. Experimental Arrangement for the Absorption Measurement .... ................ ... 15
2. Relative Quantum Efficiency of Sodium Salicylate ...... .................... ... 17
3. Spectral Intensity Distribution at the Exit Slit of the Monochromator ..... ............ 19
4. Recorder Trace of the Schumann.Runge Bands (t/- 8 to v' = 20) ...... ............. 21
5. Potential Energy Curves of Molecular Oxygen .. ... ........................... .. 23
6. Absorption Intensities of the Schumann.Runge Bands ...... ................... 25
7. The k-values of 02 in the Region From 1250 to 1750 ,.. ................. 27
8. The k-values of O in the Region From 1050 to 1350 1 .................... 28
9. Preparation and Purification System for Liquid Ozone ....... ................... 30
10. The k-values of 03 in the Region From 1050 to 2200 A ................... .32
11. Predicted and ()Obrved Elecirtoni. Transitions for 0 ........ ................... 33
12. Absorption Cure of 03 Showing Bands .......... ......................... 35
13. Potential Energy Curves of NO ........ ............................. ... 36
14. Absorption Curve of NO in the Region From 1500 to 2300 ................. 37
15. Apparent Pressure Effect of NO Bands ......... ......................... .. 38
16. Absorption Coefficients of NO in the Region From 1050 to 1600 A .............. 40
17. The k-values of COt in the Region From 1400 to 1750 A................... 43
18. The k-values of COt in the Region From 1150 to 1450 A................. ... 45
19. The k.values of COt in the Region From 1050 to 1200 .................. .46
20. Predicted Electronic Transitions for COt ......... ........................ .. 47
21. The k-values of NtO in the Region From 1080 to 1220 A .................. 49
22. The k.values of NO in the Region From 1220 to 1380 A.................. .so
12
ILLUSTRATIONS (Continued)
"23. The k-values of NO in the Region From 1380 to 1600 A.................. .. 51
24. The k-value. of NO in the Region From 1600 to 2100 A.................... 53,
25. Absorption Curve of CO in the Region From 1050 to 1650 ................ .. 57
26. The k.values of NH, in the Region From 1600 to 2200A,1 ....................... 58
27. The k.values of NH, in the Region From 1050 to 1650 A.................. .59
28. The k.values of H2O in the Region From 1250 to 1850 ................... 62.
29. The k.values of H*O in the Region From 1050 to 1250 ................... .64
30. The k-vaiues of CH4 in the Region From 1050 to 1600 .................. .65
31. Absorption Cross Sections of NO and OC ......... ......................... 68
13
ABSORPTION COEFFICIENTS OF SEVERAL
ATMOSPHERIC GASES
1. INTRODUCTION
The composition of the atmosphere in the region above the troposphere is affected by a numbet ofprocesses such as mixing, diffusion, photochemical and recombination processes. When sufficient data areavailable, it is possible to deduce reliable information on'atmospheric constituents and penetration of solarradiation, and this can be done well in advance of diect neasurements. For example, many years agoChapman (1930) made a calculation to show the existence of a transition layer of atmospheric compositiondue to the process
at an altitude of about 100 kin, but only recently direct measuremcrtts have become possible at this alti-tude. At higher altitudes the determination of atmospheric composition by direct methods becomesincreasingly difficult because of low atmospheric densities. Mnreojver, :*. thces 6cg~lis the C( .iTituents
to Loe in unstable or excited states, so that gas samples cannot be brought down to the laboratory
iu it se conditions.
Mo 9f the primary photochemical processes occurring in our atmosphere are expected to be inducedlby solar rad :ation of wavelengths below about 2800 A, which is comipletely absorbed in the atmosphere.
This 'd, ,Iear i'tom the fact that the constltuents of air are rather stable molecules that require energeticphotons to pro, ' ace photodissocistiaoi and photoionization. The importance of extreme ultraviolet radia-tion has been lo,'ig recognized in connection with the various atmosFheric layers; however, a number of
recent calculation ' oni the formation of these layers have led to diverse conclusions. For example, some of
the processes sugge ,t.'.a, !r invoked for the formation of the D layer are: photoionization by X-ramvi, photo.ionization of molecjtar oxygen in the region from 900 to 1000 1, ionization of metastable atom.- oxygenby Lyman alpha (1216 X) in xr, ýtepo, photoionization of nitric oxide in the region from 1100 .9 1300 A,
and photoionization of ji 4 - - ' , , , ,om
exercised in the choice of process illustrates the need for data on variou parameters such as the intunsity dis- 5-
tribution of the solar, extreme ultraviolet spectrum and absorption cross sections of atmospheric corntituents.
The intensity of absorption is expressed here by the absorption coefficient k in reciprocal centimeters,or by the absorption cross section, e in square centimeters. The former is defined by the equation
I - 1o exp(-kx),
where Io and I are the incident and transmitted light intensities and x (in centimeters) is the i4yer thick.ness of the absorbing gas reduced to NTP. The cross section is defined by
k
0 Maamouip received Lt publication 3 March 1953.
14
where no is the number of particles per cubic centimeter at NTP. If the k-values are known throughoutthe spectral region covered by a particular electronic transition, the oscillator strength (or f-value) of thistransition can be computed by
,,' kdv, ()
where m and e are the electronic mass and charge, c is the velocity of light and P is the wave number inreciprocal centimeters.
Absorption coefficients of several atmospheric gapes have been measured in the vacuum ultraviolet bya number of investigators. Ladenburg and van Voorhis (1933) made the first quantitative measurement ofabsorption intensity and obtained k-values for 0, in the-region from 1330 to 1670 A of the Schumann-Rungecontinuum. Schneider (1940) obtained data for dry, C02-free air at 350 wavelengths in the region from380 to 1600 A but his low k-values were only estimates. Preston (1040) determined the absorption coeffi.cients of Nu, 0,, CO, and H20 at Lyman alpha (1216 A) but disagreed with the data on O obtained byWilliams (1940) by a large factor. The absorption coefficients of C114, CO. and !1.0 in the region above1400 A were obtained by Wilkinson and Johnston '1950). More recently, similar data were obtained forNO and 14O in the region above 1400A by Mla ence (1952). Clarke (1052), Weissler, Lee and Mohr (1952)and Weissler and Lee (1952) reported data for both O and Nt at a number of wavelengths in the difficultregion below the transmission limit of LiF. At two wavelengths where comparisons can be made, theirresults agreed for 02, but not for N2. There appear to be no data for 03, CO and i113 and many wide gapsare stilf present in the available data.
For a number of reasons, measurements of absorption coefficients are more difficult in the vacuumultraviolet than at longer wavelengths. First of all, the lack of a light source with continuous s"ectrurnL:!-;nsly limits measurements of sharp bands with rotational structure. The hydrogen discharge tube,provides a contiu.•it.6-•,, to about 1650 A. Below this wavelength, available sources for intensity meas.urement give .... -, ,iectra; condensed dis-,cr.'st of the Litman type ar- too erratic for photometr%.Other inconve,'i•mcr.-,sn e inefficiency of gratings due to poor reflectivitv, scaii..t- of suitable optical materialand detectors, and the necessity of preparing gases of hit% purity.
The main purpose of this paper is to present absorption coefficients of several atmospheric gases in thespectral region from !050 to 2500 A. Most of the data have not yet been published, except a brief noteby Watanabe, Inn and Zelikoff (1952). The experimental techniques and arrangement, which are some.
- what different from those used by previous investigators, are described in detail. In addition to absorption
entsl new data obtained on the absorption spectra (bands, continua), dissociation energies and ioniza.coefiKtion t ile are presented. The application of some of the data to the theory of the formation of theD ayr,, d. eSome of the data are of a preliminarv nature, and refinement must dependo layer
is also ndey rn)e----m-
--dependluon improvement experimental tecnmiP .. waloilrlun
2. EXPERIMENTAL
Apparatus and experimental techniques used in vacuum spectroscopy are described in a book by Bomke(1937) and a review by Boyce (1941). After 1941, the activity in the field was more or less reduced to a
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attempt were ade toimpov the Execri qe n xemental arrangeme!thrA ýWT' n s usca"'ed by t hes vc
monochromator was used instead of a spectrograph so MAZ it was possible to place an absorption cell witnLiF windows at the exit slit. This arrangement greatly nminimized photodecomposition of the absorbing goaIn place of photographic plates, a phosphor-coated photunultiplia r was used as a detector; thus, the I&&~ rof analyzing the abwsorption data was greatly reduced. Tio insure! the purity of gas sampkes, each gas wW
anaiyzej with m amtsa spectrozne~er. as
The experimental arrangement employed iai the pweN, nt work. is allown in Fik'g. 1. MN1of this has been reported by Watanabe and Inn (1953); :xoweve~r, for conveieaiece, the dewcnpason of t )apparatus and experimental procedure, including some of ihoae itsed by others, are defturibed mo the fo'l fie
ing sections.
2.1. LIGHT SOURC-E
The:P_ spja~a-r to be cocaparfarveiy Wdeti pý-ogress ir. dL development of light sources suitable for phri-tometry in the vacuum ultraviolt. Lse arnd Weiseler 09~51) rtforted a low-pressure spe rk whirpi was used
-~-wk. 1 '-otoeapj mieasuremrent of absorption inte-'ý,ioi a in~ the region from 300 to ý130O A. The con-stancy of this'liglit sA-rt-is claimed to be about 2 r(, e;-nt; h wever, the source provi/dsalmt riso
'S.. -' I
po-
16
spectrum. Carpenter (1952) of Baird Associates has investigated the development* of several light sources
suitable for use in the vacuum ultraviolet. Major emphasis was in the development and construction of
high-intensity, hot-cathode, xenon lamps for use in the stud.s of photochemistry in the Schumann region.
Among the other types of lamps constructed, the ele-ctrodeless lamp excited by microwaves sl.o~ed con-
siderable promise. This type of lamp was also investigated hb Zelikoff, WN ckoff, Aschenbrand and Iomimis
(1952). They used metal vapors to obtain strong emission lines for photochemical reactions.
In the present work, the light source (D in Fig. 1) was a windowless hydrogen discharge tube with a
quartz capillary (Q) and was operated at about 600 v X 0.4 amp DC. Similar light sourc.es %ere pre' ioushld
used by Powell (1934), Little (1946), Johnson, Watanabe and Touse% (1951). Tank hydrogen was used
without purification and its flow rate was kept constant b. means of a needle val-.e and a simple flow meter
(Fin Fig. 1). The pressure in the main chamber of the monochromator was held at about I X 10-' mm Jig
when the light source was in operation. The constancy of the discharge condition was checked by the are
current, the flow rate and the ion gauge (GC). As a rule, the light intensit% did not vary more than a few
percent over a period of several hours. This was the largest source of error; however, %hen a more a,.iuratc
result was desired, repeated measurements were made at fixed wavelengths.
2.2. DETECTOR AND ITS CALIBRATION
The method of photographic photometry has been employed almost exclusiveli in the past for measure.
ment of absorption coefficients in the vacuum ultraviolet; however, Preston's experiment (1940) is an excep.
tion. He used an argon-filled platinum photocell with an LiF window for his measurement at L~rman
alpha. In view of the well-known uncertainties of photographic plates, particularly for use in the region
below 2000 A, a photoelectric method was selected.
Perhaps because of the lack of optical materials fom use as envelopes, photoelectric detection has not
been used often in the region below the transmission limit of quartz. Powell (1934), Preston (1940) and
Little (1946) used gas-filled, platinum photocells. More recently, Parkinson and Williams (1949) used a
pbosphor-coated photomultiplier to measure down to 1450 A, and Bolton and Williams (1952) down to
970 A. To find the best coating material for photomultipliers, Johnson, Watanabe and Tousey (1951)
studied the relative quantum efficiencies of a number of fluorescent materials in the region from 900 to 2500 Awith several types of photomultipliers. This work was based on energy measurements b. Packer and Lock
(1951). Their work was extended to 584 A by Watanabe and Inn (1953). G.orgy and Kingston (1951)
reported successful ute of a beryllium-copper photomultiplier without envelope in the spectral region from
50 to 1000 A. Photoelectric yield of several metals in the vacuum ultraviolet was investigated without
spectral dispersion by Kenty (1933). Baker (1,938) obtained data for photoelectric ,ield of cadmium down
to 1000 X, but his energy measurements were indirect. Recently, Ilinteregger and Watanabe (1952)
obtained the 1 .,*-lectric yield of platinum, tungsten and nickel in the region from 850 to 2000 A and at
584 A, using a vacuum moi*--hromator and direct energy measurements with a thermocouple. They ob.
tained reproducible results with windowless photocelle. Similar measurements were reported by Wainfan
* This research was supported by Contract No. AF 19(122)-432, sponsored by the Geophysics Research Directorate,
AF Cambridge Research Center.
17
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VC0 0 0 00* f 00*** 0 @0 0 00 0 o o 0 0
0
->iJI-204
-J
S~I I I I I I I I I I I I I I I I I I 1JIJI l
500 1000 1500 2000 2500
WAVELENGTH IN ANGSTROMS
Fig. 2. Relative quantum effmiency oi sodium alicylate.
and Walker (1952). The possibilities of sensitive photon counters appear promising, although the repro.
ducibility does not seem adequate at present. They have been used by Fujioka and Ito (1951), Friedman,Lichtman and Byram (1951).
In the present work, a 1P21 tube coated with sodium salicylate was used as a detector. From a num-ber of fluorescent materials tested, sodium salicylate was selected as the best material for use with a photo-multiplier. Unlike most organic fluorescent materials, it did not sublime in vacuum and remained stableover many months. Unlike every inorganic phosphor tried, its response was uniformly excellent over the
entire spectral region from 500 to 3000 .
The response of tue sodium salicylate was found linear with intensity by direct comparison with thethermocouple response at a number of wavelengths. A windowless, compensated thermocouple was used
as the radiation detector for absolute energy, i.e., as the standard for calibration of the phosphor-coatedphotomultiplier. The thermocouple response was amplified by a Liston-Becker, breaker-type amplifier
and fed into a Speedomax recorder. The thermocouple sensitivity in air, as measured with a standard lampfrom the National Bureau of Standards, was 0.076 microvolt per microwatt. The sensitivity in vacuum,however, under the usual operating conditions was found to be 1.31 p volt/u watt when helium was used
and 1.35 u volt/u watt when hydrogen ,was used in the discharge tube. A more complete description ofthe calibration was reported by Watanabe and Inn (1953).
Comparison of thermocouple and photomultiplier was made over an intensity range of a factor of 50by selecting different lines and varying the current of the light source. The largest intensity used was about2 X 10" quanta cm- sec-1. The linearity of the detector response simplified quantitative absorptionmeasurements and minimized systematic errors.
To determine the relative quuatum efficiency of sodium salicylate, type 1P21 tube was found preferableto type 5819. The latter gave a lower scattered light.to.signal ratio, as expected from the geometry of itsphotocathode. The proximity of its photocathode to the phosphor was undesirable, however, since the
18
light beam moved a little as the spectrum was scanned and a non.uniformity in the sensitivity of th. photo.
cathode became a source of error. The relative quantum efficiency was obtained by direct comparisonwith thermocouple measurements. The result is shown in Fig. 2. The ordinate is propbrtional to I,/IIdwhere 1, is the photomultiplier current, It is the thermocouple response and X is the wavelength in angstroms.The points fall nearly on a horizontal straight line, thus showing constant quantum efficiency. At the584A line of helium the quantum efficiency appears to be about 15 percent lower than at longer wavelengths.Two of the 1P21 tubes coated with sodium salicylate were stored as secondary standards, after they werecalibrated against the thermocouple under specified conditions. One of them was later used for determina-tion of the photoionization yield of nitric oxide.
In most of the absorption measurements, the photomultiplier (P in Fig. 1) was used with dynode volt-
ages from 70 to 90 volts, depending on the intensity of the beam for different spectral regions. The currentof the photomultiplier was amplified by an RCA ultrasensitive microammeter (type WV84A), and then fedinto a Speedomax recorder.
23. MONOCHROMATOR
Rather few monochromators have been built and used in the vacuum ultraviolet. Baker (1938) useda monochromator for his study of photoelectric yield. Parkinson and Williams (1949), Johnson (1952)and others have used monochromators of the type in which the grating was simply rotated about a fixedpoint on the Rowland circle. Fujioka and Ito (1951) and Baird Associates have built normal incidencemonoehromators in which the grating moves along the Rowland circle. The type manufactured by BairdAssociates was described by Tousey, Johnson, Richardson and Toran (1951).
The dispersing instrument used in the present work was a Baird 1-m (15,000 lines per inch), normal-incidence, vacuum monochromator. For the absorption measurements, a slit width of 50 microns was usedfor both slits (see Fig. 1). This corresponded to a bandwidth of 0.85 A and the resolution was about 1 A,which is believed to be the best yet obtained with a vacuum monochromator. The spectrum was scanned
at 20 A per minute and wavelengths were measured to the nearest half angstrom. Occasionally, the wave-
length reading was in error by as much as two angstroms. For thermocouple measurements, it was neces-
sary to use a wide slit (1 mm). The thermocouple was mounted directly on the slit mount which could be
rotated, and its sensitive area, 1 X 3 mm, served as the slit.
The pumping system consisted of a Kinney mechanical pump and a 6-in. oil diffusion pump. The
best vacuum obtained in the main chamber of the monochromator was about 1 X 10-6 mm Hg; normally,
it was about 2 X 10-' mm.
2.. GRATING
Gratings are, in general, inefficient in the vacuum ultraviolet, primarily because of poor reflectivity.
There is no doubt that progress in blazing technique can improve the efficiency of gratings, but the possi.
bility of finding coating materials of good reflectivity cannot be excluded.
A rather striking phenomenon which occurred during the thermocouple measurements was the great
19
1.0
0.i
BB
iC0.01
CA
800 1000 1200 1400 1600 1800 2000 2200 2400 2600
WAVELENGTH IN ANGSTROMS
"Fig. 3. Spectral intensity distribution at the exit alit of the monochromator.
increase (up to a factor of 30) in the observed spectral intensity, the greatest increase being at the shorterwavelengths. In Fig. 3, curve A represents our first thermocouple measurements of the Hs spect.rum using
1-mm slits. Curve B represents the result obtained after a major change had occurred and the observedintensity had reached a nearly stationary value. In this case, it was possible to measure the H2 spectrum
with the thermocouple down to 850 A. The 584 A line of helium was also measured. The unexpectedphenomenon was ikscribed to an increase in the grating efficiency, as a result of an unintentional coating of
the grating with platinum evaporated from the capillary of the light source. This conclusion was made
on the basis of eliminating several other possible explanations, such as changes in the detector sensitivityand light source and absorption by gases in the optical path. A proposed explanation that the improvementof the grating resulted from some sort of conditioning in the vacuum system, rather than from the coating,
seems untenable, because a second grating which received no coating did not improve while in vacuumover a period of several weeks.
In view of the fact that the improved grating has retained its efficiency, it appeared worth while to
duplicate the result. For this purpose, the second grating, which showed poor reflectivity, was sputteredwith platinum in vacuum. The first coating of about 400 1 did not improve it, but when the layer thick.ness was increased to about 1600 A, the efficiency was found to be greatly improved. The amount of scat-tered light originating from the grating did not change appreciably, and the resolution, at least in terms of
the slit width (0.05 mm) used, was not affected. The results for the two gratings (unintentional coating
20
on the first one) are shown briefly in Table 1, where R is the factor by which the grating efficiency was in-creased by platinizing. As anticipated, both gratings showed a Pomewhat reduced efficiency for wave-lengths above 1600 1. The gain made in the region from 1600 to 1100 A was rather surprising, however,since Sabine (1939) has shown that aluminum reflects better than platinum in this region. It is possiblcthat the reflectivity of aluminum in this spectral region varies widely and may be reduced by aging. Fur-thermore, it should be noted that the effective blaze angle of a grating must play an important role.
Table 1. The factor (R) by which grating effieioacy was increased by platiniuing.
Wavelength (1)' 1650 1600 1500 1400 1350 1216 1100
R ir orfitgating .......... 0.8 1.1 I.5 2 2.5 14 25
R for oreond grating......... 1.0 1.6 7 14 1 20 13
An immediate application may be made in rocket spectroscopy for the solar spectrum below 2000 A.Present efforts appear to be directed mostly to sun-followers that would provide longer exposure time.Since scattered light becomes a problem for long exposure time, it seems worth while to explore the possi-bility of improving the efficiency of spectrographs. For example, platinizcd or freshly aluminized gratingsmight be used to photograph the solar extreme ultraviolet.
2.5. ABSOitPTION CELL AND GAS-FILLING SYSTEM
In the region down to the transmission limit of LiF, it is preferable to enclose the absorbing gas in a cell,rather than to use the spectrograph chamber which is usually in communication with the light source. Inthe former case, pressures of the absoi sing gas can be more readily controlled over a wide range, and con-tamination of the absorbing gas, as well as changes in the spectral characteristics of the light source andgrating reflectivity, can be minimized. By varying the pressure in the cell, it is possible to measure absorp-tion intensity covering a range of 10 or more.
All of the investigators referred to in Section 1 placed their absorption cell between the litht source andthe entrance slit, or filled their spectrograph with the absorbing gas. As observed by Ladenburg andvan Voorhis (1933), photochemical change of the absorbing gas is quite possible when the cell is near thelight source. Moreover, the transmission of the ce|l window may change because of sputtering and forma-tion of color centers. Therefore, the absorption cell was placed at the exit slit where the number of photonspassing through per hour was estimated from our thermocouple data to be at least a few orders of magnitudeless than the number of molecules. A pyrex cell (diameter 2.5 cm, and length 4.7 cm) with LiF windowswas used for the absorption measurement. For some photoionization measurements a similar cell (6.8 cmlong) with electrodes made of parallel strips of platinum was used. The platinum strips were located nearthe inner wall of the cell and outside the light beam emerging from the exit slit. Electrical leads were?rovided from the platinum strips to the external measuring circuit. For both cells, cleaved LiF platesabout 1 mm thick were used instead of lenses, since the divergence of the beam was small and good trans-mission was desirable.
2J
V 89
10 13Schumann-Runge Bonds 12 14
a• at 74.5mm Hg 15 20
16
WAVELENGTH (A) 18/
i 850 1800 1750i n 1 I I IFig. 4. Recorder trace of the Schumann.Runge bands (i' - 8 to V - 20).
The gas.filling system (see Fig. 1) was in direct communication with the absorption cell. The systemwas provided with two McLeod gauges (MI and M2), an Alphatron gauge, mercur7 manometer, and an ion
gauge (Gt). Provisions were made for removal of gas samples for massopectrometric anallysis. Some ofthe details are not shown in Fig. 1.
2.6. PROCEDURE
After purification of the gas to be studied, the experimental procedure w" as follows. First, a sampleof gas was removed directly from the gas-filling system and analyzed for possible contamination using aConsolidated Mass Spectrometer (Model 21-103). Second, the absorption cell was evacuated to about10-6 mm ITg and by scanning the H, spectnun, the spectral intensity was obtaintd as a recorder trace ofthe photomultiplier current. Third, the cell was filled with the absorbing gas at a suitable pressure andthe photomultiplier response of the transmitted light was recorded on the same trace using an ink oi different
color. The second and third steps of the procedure were repeated using various pressures to cover theabsorption intensities and to determine whether Beer's law hold.
To obtain the absorption coefficients from the recorder traces, corrections were made whenever neces.sary for the scattered light and the fluorescence of tie LiF windows. These were, as a rule, small correc.tions. Several hundred wavelengths, 1 to S . apart, were used in most cases and for each wavelength datawere obtained for several pressures over a range of at least ten.
In order to interpret the absorption data, it is necessary to have some idea of the nature of the absrp-tion spectrum, i.e., whether the absorption spectrum is a continuum, a diffuse band or a sharp hand. Fora continuum or a very diffuse hand, high resolution is not required and, as a rule, Beer's law holds at rela-tively low pressures. On the other hand, the resolution we have used was not sufficient to give accuratek-valuea for sharp bands and the data showed an apparent pressure effect. Absorption laws have beenreviewed, for example, by Nielsen, Thorntov *nd Dale (1944).
Figure 4 represents a recorder trace of a partially resolved spectrum of the Schumann-Runge bands.The curve designated Is represents a portion of the hydrogen continuum. It can be seen that the resolutionwas not adequate to resolve the P and R branches of the rotational structure. For this type of band, theapparent k-values varied with pressure. Nevertheless, these values give some idea of the magnitude ofabsorption intensity and may be useful for semi-quantitative applications and for detection of intensity
anomalies in a band progression. In the following discussions, regions of various spectra where the dataare semi-quantitative will be pointed out.
Purification of gases and other items pertaining to a particular gas will he described in conjunctionwith the result and discussion of the gas.
3. OXYGEN
A!r',•,n coefficients of oxygen have been measured more extensively than those of any other gas(see Section 1). However, to make the data more complete it would be desirable (a) to obtain values inthe region from 1670 to 1900 .; (b) to fill a number of gaps in the shorter wavelength regions; and (c) toestablish the value at Lyman alpha.
The oxygen sample (Linde Air Company) used in this measurement required no purification. Mass-spectrometric analysis showed about 0.1 percent impurity, but this was mostly nitrogen which is quitetransparent in the region of investigation. Pressures from 0.1 to 500 mm Hg were used.
For convenience ( ,iscussion, the results are divided into three spectral regions: the first region extendsfrom about 1900 A to the convergence limit of the Schumann.Runge bands which are vibrational bandssuperimposed on what appears to be a continuun; the second covers the Schumann-Runge dissociationcontinuum; and the third includes a number of diffuse and sharp bands separated by rather transparentwindows. Papers on the absorption spectrum of oxygen in these regions by Price and Collins (1935),Knauss and Ballard (1935), Hopfield (1946) and Tanaka (1952) have provided helpful information as tosome of the details of the spectrum which were not resolved by our equipment. Figure 5 represents thepotential energy curves of the well-known electronic states of molecular oxygen.
ev
I0-s 3o
863
3+. +
E 0
02 E
x~9
Fig. S. Poute ntialerg curve of molecular oxygen.
25
9 8
0I-
zWI 7
0LL
LL 6W0 5U
z0
-0.1Q.- 4
00
SI I !
o~,IiI I1750 1800 1850 1900
WAVELENGTH (A)
mg. 6.Ablptioo intensities of the Schmann.-Run- e bands.
The absorption intensities of the Schumsnn.Runge bands have been calculated theoretically by Pillow(1950). In Table 2, these are compared with the results of this work. For the experimental data, the
absorption inten:tien are relative values with that of the 12,0 band arbitrarily taken as 100. The oxygen
pressure was 7.8 mm Hg And this pressure was too low for observation of bands below 8,0.
Table 2. Intansities of the Schwnann.ltunge bands.
Band 5,0) 6,0 7,0 8,0 9,0 10,0 11,0 12,0 13,0 14,0 15,0 16,0 17,0 18,0
Theoretical ...... 1 5 20 29 S1 50 78 100 90 95
Ezpe tine..ta 19 42 59 90 100 87 124 146 161 166 160
Considering the type of data being compared, the agreement is rather good; however, data obtained at higher
pressures show that the decrease of intensity is less rapid from 6,0 to 4,0 than the evaluation by Pinlow.
26
3.2. RESULTS FOR THE SPECTRAL REGION FROM 1750 TO 1300 X
For the Schumann-Runge dissociation continuum I I.- -. 12;#, the important absorption measurement
of Ladeaburg and van Voorhis (1933) has been generally accepted and applied to atmospheric absorption.
Their result showed a maximum absorption (k about 490 cm-1) at 1450 A with a nearly symmetrical intensity
distribution. This is consistent with the dispersion measurement made earlier by Ladenburg and Wolfsohn
(1932). The index of refraction of oxygen, obtained experimentally, was expressed by three resonance
wavelengths and one of them (At = 1468 X) was ascribed to the Schumann.Runge continuum. Still earlier,
Koch (1912) had obtained a dispersion equation with As - 1421 At.
By varying the pressure over a wider range (0.1 to 50 mm), it was possible to extend the work of Laden-
burg and van Voorhis to both longer and shorter wavelengths, and the result is shown in Fig. 7 by the solid
curve which represents the mean of several hundred points. Within experimental error (about 5 percent),Beer's law held over a pressure range of a factor of 20. The result of Ladenburg and van Voorhis is shown
by the dashed curve which is considerably higher over the major part of this region. The maximum in the
present observation is rather flat and is 380 cm-' at about 1420 A (this wavelength agreeing with the valuegiven by Koch). The intensity distribution is rather asymmetric, the short wavelength side being fairlysteep, and there may be a discontinuity at about 1400 A due to an interaction of potential curves (B"Z , andan as yet unidentified curve). Although Steuckelberg (1932) obtained theoretically an intensity distribu.tion curve agreeing fairly well with the data of Ladenburg and van Voorhis, it must be recalled that hiscalculation depended partly on their experimental data.
Since the discrepancy shown in Fig. 7 appears to exceed the combined experimental error, a further
check was desirable and was kindly provided by Jones, Taylor and Greenshields (1952). Using a Bairdmonochromator, but an RCA quartz photomultiplier, they measured down to the limit of quartz trans.mission, and our result agrees with theirs at least in the region from 1750 to 1570 A (the lower wavelengthbeing their limit). The following explanation might be made regarding the photometry used by Ladenburg
and van Voorhis. It is reasonable to suppose that the fixed sector disks used by them for intensity calibra-tion reduced the sizable amount of scattered light, as well as the intensity of desired wavelengths. In thiscase, the calibration data may not be directly applicable to lines which have been greatly reduced in intensity
because of the absorption of 02, since 02 itself would probably not have reduced the amount of scattered light,at least not so much as did the sector disk. This argument is based on the "fact" that scattered light ina spectrograph consists mostly of wavelengths longer than 2000 At.
Three minor maxima at 1293, 1332 and 1352 A were observed on the short wavelength slope of the
continuum. These were recent!y found photographically by Tanaka (1952) who used a Lyman continuumas source and varied the pressure of 02. These bands or continua apparently escaped photographic detec-tion for many years, since they are superposed on the slope of the strong continuum. Actually, by lookingfor them one can see them in a published photograph by Hopfield (1946). They were readily observed withthe phosphor-coated photomultiplier, even using the many-lined spectrum of II1 as source. According toTanaka, these bands appear as continua with a 3-m grazing-incidence spectrograph and the wavelengthsof the maxima could not be measured precisely. He has suggested that the first "continuum" at 1293 Aleads to the dissociation products IP + 'S and the other two to ID + ID or IP + IS (see Fig. 5).
Using Eq. (1), thef-value of 0.161 was obtained for the Schumann-Runge continuum (1290 to 1750 A).This value is somewhat smaller than the value (0.193) obtained by Ladenburg and van Voorhis.
27
600
0100
z
LALILLJ0
z0
0~
0 10
"21300 1400 1500 1600 1700
WAVELENGTH (A)
A&F 7. Th hyalu o % in the NO= from 1 o 10
28
o000 I.. I
2 ioo-10-
ZW0U_ F
I.-
"W 10- -
z E
a-OrA 80UI)
> Lyman a
0.I1050 1100 1150 1200 1250 1300
WAVELENGTH (A)
Fig. 8. The k-values of O0 in the region from 1050 to 1350 A.
3.3. RESULTS FOR THE SPECTRAL REGION FROM 1300 TO 1050 A
Molecular oxygen has a complex absorption spectrum in this region and most of the bands still need tobe classified. This region is of great importance to the study of the upper atmosphere, since solar radiationcan penetrate down to the D layer through the several windows of the Ot spectrum. For example, Lyman
alpha lies in one of the deepest windows and its enhancement during chromospheric eruption may very,well provide the necessary energy to account for the increased ionization and radio fadeout. Furthermore,
it is important to know whether dissociation of 02 by photons in this region of the spectrum supplies an
appreciable amount of metastable oxygen atoms.
At Lyman alpha the absorption coefficient was found to be 0.30 cm-1 at pressures of 70 -m Hg and0.50 cm-1 at 490 mm. These values are in close agreement with the data obtained by Preston (1940) whoused pressures from 30 to 290 mm and a cell length of 21.8 cm. Since he observed a pressure effect, he
extrapolated his data to zero pressure and obtained k - 0.28 cm-1. Our extrapolated value was 0.27 cm-.Incidentally, absorption coefficients obtained by the present method for N2, CO2 and H20 at Lyman alphaagreed remarkably well with those obtained by Preston.
Because of the complexity of the 0, spectrum and the fact that a many-lined spectrum rather than acontinuum was used as source, some of the data in this region are not reliable. In the region below 1170A
the absorption bands show sharp, fine structure as well at sortie diffitierrei,. The "coefficient" shown ii)Fig. 8 for the poorly resolved region front 1060 to 1170 A must therefore be considered semni-'.jiantitativ..As in the ease of the Schunann-Rtinge landid, the mainima are probably tpoo low and the mninija too high,although the apparent pressure effect was somewhat less. Included in tile paper by "eisslhr and Le' (1952)
are k-values at 7 wavelengthas in this region. These are, i n ge neral, considerably la rger. and may i" explai nedI
by the fact that they used higher resolution. Although close comparisnn may not he-jnwtified. S'hiaeidcr's
values (1940) for this region seem to he, in a rough way, lower than those obtained by Weissler anti ".e.
In Fig. 8. the bands A. 11, C. D. E and F' are vcr diffuse with hardly any structure, even when obsoervedwith D 3-m grazing-incidence spectrograph. Price and Collins (1935) who observed nis•t of them atcribedthe diffuseness to predissoiiation. This was confirmed 1.% Tanaka (1952). who stitilied the spectrum ingreater detail. The latter also relprted two rew progressions. one (if which was suggested ito lIw a memberof the Rydberg series converging to the first ionization piotcntial. A few inemher• ,,f lhese progres.sionsneem to appear in our data (Fig. 8). Because of the gencra! diTfusene-s of the slpetruni. flu" k-alu••s shown
in Fig. 8 for the region from 1170 to 1300 A should be fairly rcliablh.
Price and Collins (1935) also observed a weak continumn at 1103 A. cAtcnding toioard shorter wale-Zengths. The present measurements support their observation; howerver. its intensity (about 2 cmi-' inFig. 5) is much less than the estimate (50 to 100 cr-') reported hlo Weissler and Lee ('J952).
4. OZONE
Many investigators have studied the ozone niiolctle byv ljth.1al method.s to deterine its. t hiolei'lar
structure and to obtain quantitative data on its ah•sorption sp.etr.mtrmi. pa•rtiicularly ill roite,,t ion with til.study of atmospheric ozone. As a result, a number of haunds and continua have be.n found iii the uhtra-violet, visible and infrared regions, and although ozone is somewhat unstable, a sizable uamotnt of reliabledata on absorption coefficients and. flects of temperature and pressure ;s now available. Tl'hes.e. howe~er.
will not be reviewed here.
In the vacuum ultraviolet region, the only experimental i•t,,stigation aplicars to li that made i,v ['rice
and Simpson (1941). According to them the reuhlt was °disappointing." since the photographic plates
showed no strong absorption bahds in the region from 2300 to 1600 A. In tile region below 1600 • a ltrongcontinuum appears in the reproduction of their photographic plate. They stated, however, that in thisregion the absorption of O resulting from the photodissociation of ozone prevents further investigation.
To determine the absorption spectrum and the k-values of ozone in the vacuum ultraviolet, it is nieces-
sary to use very pure ozone. Since ozone decomposes readily and is chemically reactive, it is almost ima-
possible to obtain pure ozone. If the amount of impurity is small, however, corrections may be made by
using the absorption coefficients oi the gases appearing as impurities. in this way, it was possible to obtain
absorption coefficients of ozone from 2200 to 1050 A.
* 7T• work was done in collaboration with Dr. Y. Tanaka. Hias extension of the work to ioiger wayvrengths will e rclportedseparately by ham.
Co,.t ~ Copy
30
SOLUT ION
FOR CHEMICAL-' ANALYSIS
H SO 4MANOMETER
PURIFICATION
TOOXYGEN TANK C 1
M~NOETE -~ UMPSAMPLE
LIQUID- OZONIZERNITROGEN
Fig. 9. Preparation and purification system for liquid ozone.
4.1. PREPARATION OF OZONE AND CONDITIONS OF ABSORPTION MEASUREMENT
Figure 9 shows schematically the apparatus used in the preparation and purification of ozone. Tankoxygen was passed through a long CaCIh drying tube and then through a liquid nitrogen trap, thus removing
H,0 and CO which might be present as impurities. The oxygen was then passed into the ozonizer, consist-ing of a double-wall pyrex tube, the inside of which was filled with aqueous NaCI and the outer wall wrappedin thin aluminum foil. A 15.kv, 450-va transformer was used for the excitation. Liquid ozone was col-lected in liquid nitrogen traps. Since oxygen is the main impurity, the gas pressure in the ozonizer was
kept below 12 cm Hg to prevent condensation of oxygen, together with ozone. After collecting a few cc of
liquid, the process was stopped and the system pumped for about half an hour, until the pressure decreased
to about 0.1 mm Hg (the vapor pressure of Ox at liquid 02 temperature). (See Jenkins and Birdsall (1952).)
If nitrogen is present in tank oxygen, the ozonizer would produce nitrogen oxides that would condense
with ozone. Since these oxides have strong absorption in the Schumann region, their presence could not
be tolerated. Accordingly, the ozone samples chosen for study were obtained by several fractional distilla-
tions of the liquid, retaining only the middle portion each time.
31
To fill the absorption cell, liquid ozone was transferred to the gas-filling system. The transfer wasaccomplished without any explosions by keeping the ozone cooled in liquid nitrogen until just prior to theabsorption measurements. The liquid was allowed to evaporate slowly into the cell and the pressure meas-ured with a U-type. HS,04 manometer. For pressure measurements below 1 mm Hg, an expansion methodwas used. The vapor pressure of H2SO4 at room temperature did not produce any noticeable absorptionas was shown by a blank run. Since ozone decomposes at an appreciable rate, the duration of each runwas limited to five minutes, (luring which time a range of 100 A could be covered. For each wavelengthregion, four or five different pressures were used. Pressures from 0.2 to 25 mm Hg in the absorption cellwere used to cover the absorption intensities.
During the filling of the absorption cell (previously evacuated to "-10-' mm Hg) an interesting phenom-enon wis observed. As the liquid ozone evaporated into the cell, a sudden increase in photomultiplier cur-rent, reaching a maximum value in a few seconds, was observed. The current then decayed to its originalvalue. Upon pumping out the ozone, the photomultiplier current again increased and decreased, although,
in this case, the rate of current change was slower and the maximum was much less. This phenomenonoccurred both with the light source on and off, and may be due to chemiluminescence on the inside of thecell. No doubt some ozone decomposed in this process, giving rise to oxygen impurity, but the amountwas small as determined subsequently from the absorption data.
4.2. DETERMINATION OF PURITY
The purity of the ozone sample was determined by a combination of three methods. First, the vapor-ized ozone was allowed to react with aqueous KI and the liberated 1: titrated with standard Na2S.20. Herethe ozone was found to be 90 to 95 percent pure for all samples taken from the purification system and fromthe gas-filling system. To determine the amount of nitrogen oxides, a Consolidated Engineering Corpora-tion analytical mass spectrometer was used. The analysis showed no detectable amount of nitrogen oxidesin the ozone sample, indicating that the nitrogen oxide content was less than 0.05 percent. For this analysis,the sample was allowed to stand several days to convert the ozone to oxygen, since ozone may attack themetal parts of the mass spectrometer. Under this condition, the analysis for CO, CO, H20 and hydro.carbons would result in high values, because their formation by the action of ozone on stopcock grease wouldbe exaggerated by the length of the exposure time. Therefore, an analysis of these gases was not made.The absorption measurement itself served as the third method for estimating the amount of impurities.
This was made possible by the data for a number of gases obtained previously with the same absorption
apparatus. There are three strong peaks for COM, at 1088, 1119 and 1129 A, with k-values of 3600, 4400and 2500 cm-1, respectively, and these appeared on the uncorrected absorption curve of O. The data showed
that the amount of CO was between I and 2 percent. Similarly, from a strong oxygen peak (k = 940 cm-')at 1243 X, the amount of 0, in the absorption cell was found to be about 7 percent. The presence of CO
was indicated by five weak but sharp peaks which corresponded to the heads of the CO Fourth Positivebands from (0,0) to (4,0). The amount of CO was estimated to be a few percent.
Although the chemical method showed that the total amount of impurities was less than 10 percent,
the absorption data indicated that the value was somewhat higher, at least in the absorption cell. This is
quite possible, since the reaction described above can provide the additional impurities. In general, the
absorption data showed that when relatively high ozone pressures v ire used, the percentage of impurity
32
1000
79
0ii
Z 0
10 00
g ill I
I- 000 120 14010 80 0020
ZW EIS
A, I1
o III fil
o II
o 00 12001 14010 80 0020
WAVELENGTH IN ANGSTROMSFig. 10. The k-valuee of 0. in the region from 1050 to 2200 A.
was low,more nearly agreeing with the chemical determination. Therefore, to obtain the corrected absorp-
tion intensity curve for ozone, only the data obtained with the highest pressures were used. In view of all
these uncertainties, it is estimated that the combined experimental error would be as high as 10 percent.
4.3. RESULTS AND DISCUSSIONS
The solid curve in Fig. 10 represents the absorption curve of Os. This curve was obtained after correc-tions for 02, CO and CO2 were applied to the original data. It cannot be an absorption curve of an unknown
impurity with very high absorption coefficients (order of 101 cm-1), since the result was reproducible in
spite of the fact that the various observed impurities varied considerably for different runs. The absorp-tion curve (dashed) for Os, the main impurity, is included for comparison.
33
ORBITALS IONIZATIONPOTENTIAL
4b12 4.51(o 2 p) 0
3b \ 00.8(v 2P)0
10 a2 I(w 2 p)0
5 0 i I 12.5 (2P,),-2 2
2 b-13.01(2 PxLv b- , KI3.5 (2 P_)_
2 b
I b 16.5 (ru2p )2
42 17.5 (ru2 p) 2
2o 17.5C (-2p) 2
3a 2
'MAX LR. VIS. 2550A ~ 2000A
INTENSITY ww w (vw) (vw) (f) s (s)
Fig. 11. Predicted and observed electronic transitions for Os.
34
For convenience of discussion, the regions above and below about 1300 A will be considered separately. pAbove 1300 A there appear to be continua with maxima at 1725, 1450 and 1330 1. The 1725 A continuum/merges at about 2000 A with the know%, strong continuum which has a maximum at 2550 A. The regienfrom 2020 to 2200 A, shown in Fig. 10, overlaps the regikn studied by Ny Tsi.Ze and Choong Shin r jaw(1933) and by Vassy (1941), who measured down to 2135 and 2020 A, respectively. The values 'in thisregion were found to be somewhat smaller than their values. The difference was about 10 percent at 2200Aand about 40 percent at 2020 A as compared to the data reported by Vassy. The discrepartcy at about2020 A appears to be greater than the combinec experimental error. It is difficult to asses, the erroas sincethe experimental conditions are quite different: the use of ozonized oxygen, photogrAphic photometry,quartz spectograph, and absorption cell placed before the entrance slit against the ut t of purer ozone, linearphotoelectric photometry, vacuum mo-aochromator, and absorption cell at the exil, sift. It is possible thattoo small a correction was applied tv the present data and this may account for. "ibout 10 percent. A furtherinvestigation seems desirable.
Mulliken (1942) has pr, posed an electronic configuration for ozo•e and Fig. 11 shows schematicallyhis results, all notations being his, where the solid arrows indicate o'served transitions and dashed arrowsindicate other predicted, transitions. The strong band, 4a,2 - 3o, predicted in the region just below2000 A was not observed in the present study. The 1725 A eontinuum seems to correspond energeticallywith it but is much weaker than predicted. Moreover, ow, results agree with those of Price and Simpson(1941) who reportd no bands in the region from 2300 t- 1600 A. Our absorption coefficient at 1600 A is35 cm-', graduvIv- decreasing to a minimum at about ,"0 A . Energetically, the 3a,'-3b,° and 50as-W4bstransitions c .' "-Nond very roughly to the observedi 1725 and 1450 A continua, respectively, although thetransiticý. pro2abilities are not known. It might also be noted that the three strong continua at 1120, 1215and 1330 .)ý'A"Arespond fairly well to the energy differences among the orbitals 3ail, 4asl and lb,2.
An element of uncertainty is introduced into these theoretical considerations since Mu~likea adoptedan acute apex angle (390) for Os. Recent infrared study by Sutherland and Penny (1936) ,and electron.diffraction measurement by Shand and Spurr (1943) seem to support an obtuse apex angle (1270). Withouttaking account of intensity relationships, the following transitions for the case of obtuse apex angle appearto correspond energetically with the observed continua.
Continuum Maximum Attempted Trr.nsition
1725 3a, -+ 2b,1450 2,"* 5a,1330 2s 5a,1215 las -4 462
or 1" 5a
Figure 12 presents the ozone absorption spectrum foi- the region from 1300 to 1060 A showing moredetails. Because of the weakness of the bands as compared to the continuum, it is difficult to determineprogressions. However, there may be some progresarAons that have separations of ,--•600 and '-'800 cm-'.The continua having maxima at 1215 and 1120 Aa re similar in shape and intensity to that at 1330A. It ispossible that there are two other continua at ] i65 and 1070 X but the former does not have a prominentpeak and the latter approaches the transp&,,tncy limit of the LiF window, making it difficult to classify
with certainty.
35
LA- 500 00
100
1100 1150 1200 1250 1300 !3-,-N
WAVELENGTH (A)
Fig. 12. Absorption curve of Os showing bands.
It is interesting to note that ozone has a high absorption coefficient at Lyman alpha where most of thegases reported present in the upper atmosphere have comparatively low coefficients, oxygen and nitrorc.. mai
particular. Water vapor is ane extption to this with a k-value of 387 cm-1. In the region of the Schumann-
Runge continuum, the absorption intensity of ozore is almost as strong as that of oxygen. Furthermore,the intensity of the S.R continuum decreases rapidly on the long wavelength side, whereas, for ozone the
decrease is very gradual. The absorption curves cross at about 1700 A (k - 20 cm-1).
It is apparent from the preceding discussion that further investigations are required to determine the
electronic states of the ozone molecule. It is also desirable to measure the ionization potential, since Wulf
and Deming (1938) have suggested that the D layer might be produced by the photoionization of o~zone.
5. NITRIC OXIDE *
The presence of nitric oxide in the high atmosphere is not yet established. Laboratory experiments
do show that this gas can be produced photochemically from NtO, a known atmospheric gas, but the chemical
process is not known with certainty. The formation of NO by other processes, such as the association of
atomic oxygen and atomic nitrogen, have been proposed; however, quantitative data are extremely meager.
Nevertheless, it is probable that NO is present in the atmosphere, and its importance would depend on its
amount and distribution. In anticipation of the importance of NO, absorption data for this gas have
been included.
The absorption spectrum of NO in the vacuum ultraviolet has been studied by Leifson (1926), Lambrey
(1930), Tanaka (1949), Migeotte and Rosen (1950). Their data and those obtained from various emission
spectra of NO have provided the basis for the electronic states of this molecule. These states are summar-
ized in the energy diagram, Fig. 13. Although tile dissociation energy is here shown as 6.48 ev, this value is
still the subject of controversy as in the case of N2.
0F. F. Marmo covitributed a major part in this work. A more complete report will be published by him in J. Ope. Sac. Amer.
36
IV NO*14 I- NO +
s4U S9 10.680v (1061A)
0 Pu 3P 0o.os (1232A)
Et ..1.3p I st Ionization Potential (9.5@v) 1306o
-u 2D 8 .0V (0397A)
.45 (0466A)
8 -- CYs-- 'D
E// -I -I E'I 7.5 4 ev
,/;i; wTI". 4S 3 p D 6.60/,; "-- _ •+ '6.46 (1911A) -
\A / s 2 Is G-.496 a r/D2IT 5.63
K~N 0 A - 5.-4' 72 2S 2 2 P3 1lst2s22 P4
4 TTL
2 x'T T NO
1.0 2.0Ar
Fig. 13. Potential energy curves of NO.
The ultraviolet absorption bands of NO which have been classified lie mainlh in the region from 1500
to 2300 A. Very little is known at lower wavelengths besides the three R.vdberg series observed by Tanaka
(1942) in the region below 1000 A. The energy diagram of NO, however, suggests a number of possibilities
in the region from 1000 to 1500 A. There are possibilities of dissociation continua as well as band spectra.
Furthermore, the first ionization potential is 9.4 ±0.2 ev (or about 1320 A) according to electron impact
measurements by Hagstrum (1951) but no Rydberg series nor ionization continuum have been identified.
The only measurement of absorption coefficients of NO in the vacuum ultraviolet is that due to Mayence
(1952). She made photographic absorption measurements using a grating spectrograph down to about
1400 A, and reported a weak continuum with a maximum at 1470 A. In the region below 1400 A, there
appear to be no published data on k-values.
5.1. PURIFICATION OF NO
In order to obtain reliable quantitative data for NO in the Schumann region, gas samples of high purity
are ,-Iquired. Most of the gases which are possible impurities, have rather high k-values in this region.
As described below, it is not safe to assume the purity of this gas even after careful purification.
37
2 I 050- 0
5 4 3 2 I 0
20- 9 8 7 6 5 .,L+)
zI-..2. ABSORPTION OF NO0"30-C-)0
0
zCLt i--
1500 1700 1900 2100 2300
WAVELENGTH A (k)
Fig. 14. Absorption cukve of NO in the retion from 1500 to 2300 AL
Nitric oxide was purified from tank gas by a method similar to that used by Jobhston and Giauque
(1929). In spite of the fact that the first purified sample was carefully obtained, a mass spectrometric
analysis revealed almost 3 percent of NIO. Preliminary absorption data, however, were obtained with
this sample. Then the first sample was subjected to repeated fractional distillation until the total impurity
was less than 0.05 percent. The preliminary and the final absorption curves showed a conspicuous differ-
ence: the former hada definite maximum of continuous absorption at 1285 A where the final curve appeared
flat. In fact an application of the absorption coefficient of NO) at this wavelength indicated 3 percent of
N20, in agreement with the mass spectrometric analysis.
5.2. RESULTS FOR BAND SYSTEMS
The absorption spectrum of NO in the region from 1400 to 2300 1 consists of several systems of sharp
bands which include the #, ,, A and e bands (see Figs. 13 and 14). There is very little overlapping of these
bands above about 1900 1. At lower wavelengths, however, the spectrum is very complex, with
many overlapping bands and the classification of bands in the region from 1400 to 1700 A is still rather
incomplete.
The resolution (about 1 A) used in the present experiment was not adequate to resolve the fine structure
of the various bands and so the measured k-values are only semiquantitative. Figi. 14 represents the
38
ABSORPTION OF NO BANDS
0-9 1 -0 'd 5
100033
0.33.0.3.
I-- O.4•..4W
- 10- 13..2U.
A. 1.334.00- 400 _
0 41400
?go- .90.
o 146146. 8-I-0
. 1 / 7900
0S1 11 .
50.0-. so0.0
_o 101 -k
1 65 1900 1905 1910 1915 1985 1990 2040 2046 2050
WAVELENGTH (A')
Fig. 15. Apparent pressure effect of NO bands.
apparent absorption intensity of the NO spectrum in the region from 1500 to 2300 A obtained with a pres-sure of 14.8 mm Hg in the absorption cell (47 mm). The position of the 6, -y, 8 and e bands are indicatedat the top. As in the case of the Schumann-Runge bands of 0, the measured k-values showed a strong,apparent presaure effect which might be attributed entirely to the lack of resolution, although a true pres.sure effect, particularly at high pressure, is not excluded. The ae.parent pressure effect is shown in Fig. 15for four bands (8-9, 8.0, 0-5, y-2 ). The number adjacent to each curve is the pressure (in mm Hg) of NOwhich was varied from 0.33 to 101 mm to cover the absorption intensities in the region from 1500 to 2300 A.All the observed bands in this region showed a similar effect: as the pressure is increased, the k.value decreasesrapidly at first and then more slowly so that, in some cases, it even appears to approach a limit.
Mayence (1952), who used a spectrograph with about the same resolution, found that, for at least someof the P bands, the k-value was independent of pressure in spite of the lack of resolution. The pressure
that was used, however, covered a comparatively narrow range (about a factor of 5), and the product ofcell length and pressure was about one order of magnitude greater than the largest product used in the
present work. For example, for a cell length of 500 ram, Mayence obtained the following k-values for theshorter wavelength peak of the P-5 band: 0.21 cm-1 for a pressure of 60 mm Hg and 0.17 cm-' for a pres.sure of 200 mm. These values are not strictly constant and they appear to be consistent with the data for
39
the #-5 band shown in Fig. 15 which was obtained with a cell length of 47 rnm. Thiu, it is suggested that
the conclusion made by Mayence might not be justified, at leasi tir the lower pressures that she did not use.It may be added that Mayence found that k-values varied ilivericly ab the square root of cell length for
lengths from I to 500 mm.
As in the case of the Schumann-Runge bands, the k-values for the NO bands, although semiqnantita.tive, may be applied to absorption measurements obtained with another optical system with equivalent
resolution. This may be of importance in rocket spel troscopy to determine the amount of atmospheric NO.Furthermore, the absorption data may be useful for measurements of relative absorption intensities within
a given band system.
The relative intensity distribution of the 3y bands was found to be in good agreement with that ofMayence. The well-known intensity anomaly was clearly noted at the 4,0 band (see Fig. 14). Whetherthis anomaly is due to the coincidence of the y' (4,0) and the 1 (0,0) band or is a true anomaly within the,y systm could not be established.
Many bands belonging to the (V/,0) progression of the weakly absorbing f bands are either superimposedor partially overlapped by strong 6 and y' bands. Therefore, very little can be deduced regarding the inten-sity distribution of the # bands. There seems to be a regular increase in intensity from v' = 3 to e/ = 10,
although v' - 7 and V' - 8 are overlapped by other bands. For v' > 10 there is a gradual decrease. Thisresult, as well as that of Mayence, seems to indicate no intensity anomaly up to v' = 11.
The relative intensity distribution of the three known 6 bands (v' = 0,1,2) obtained in this experimentis in accord with that of Tanaka (1949) and Migeotte and Rosen (1950). No bands of the 6 system withv' > 2 have been observed in absorption, and Tanaka explained this phenomenon as due to a strong inter-action of the C IZ state (see Fig. 13) with some unknown state of NO.
A number of diffuse bands were found in the region below about 1400 A (see Fig. 16). These bandsare comparatively weak and are superimposed on a moderately strong continuum. Their distribution andintensities were so irregular that it was not possible to recognize any progression; however, their diffuseness
suggests predissociation or preionization. Unlike the case of the bands at longer wavelengths, the k-valuesfor these bands were independent of pressure and, therefore, should be reliable.
5.3. ABSORPTION CONTINUA
Mayence (1952) reported a weak continuum with a maximum at 1470 A and ascribed it to the transi-
tion 211 --+ 2HI of NO, using 5.29 ev as the dissociation energy of NO. Figure 15 does show that there might
be a continuum underlying the bands in the region from 1400 to 1600,k. In this region, however, the vibra-tional bands are intense and closely spaced; hence. it is not safe to draw a conclusion from data obtainedwith relatively poor resolution. In fact, the k-values for the several minima in this region show an apparentpressure effect which may very well be due to lack of resolution. The k-values of several minima for severaldifferent pressures are shown in Table 3. A few values at shorte." wavelengths, where there appears to be
a true continuum, are also included for comparison. The k-values are fairly constant for the first three
wavelengths, but this is not so for the minima at about 1389, 1419, 1469, and 1514 A. Thus, from this data
it is not possible to conclude that there is a continuum with a maximum at about 1470 A.
40
100-
z
w 40
I.-
0 1
C NO0Co
4
I I I I1100 1200 1300 14(jJ 1500 1600
WAVELENGTH (A)
Fig. 16. Al•eotption coecenta of NO in the region from 1050 to 1600 o.
Table 3. Some k-valuen (en-') in the region from 1332 to 1514 Aor several pmreinuee
1TPam g 0.52 0.96 192 472 7.15 10.1 232 33 100
1332 ........... 44 45 42 46 4,5
1343 .......... 38 41 43 43 U 43
l3M ........... 32 31 34 32 31 32
1389 .......... 7.4 7.5 7. 1 6.8 6.9 5,7
1419 .......... 12.9 10.7 9.6 8.5 8.1 S.9
1469 .......... 7.1 6.5 6.2 5.9 4.5 4.2 2.85
1514 .......... 5.4 4.9 4.4 3.9 2.85 2.39
As shown in Fig. 16, there is a continuum in the region below about 1380 1 which has not been reportedpreviously. Except for a few bands superimposed on the continuum near 1350 A, the k-values were inde.pendent of pressure (factor of 20). The weak bands in the region below 1300 A also followed Beer's law.
41
The continuum itself appears rather flat and may be due to overlapping of more than one continuum.
The long wavelength end of the continuum is very close to the energy of dissociation products N (ID) and
0('P), so that it is plausible to attribute this continuum to the transition from the ground state to one of
the allowable upper states deduced from these products by the Wigner-Witmer rule. There are several
such states, including the upper state of the 0 bands. A rough extrapolation of the intensity curve, plotted
on a linear scale from 1070 A to the long wavelength side, terminates near 1230 X, the energy of dissociation
product N('P) + 0('P); so an analogous interpretation can be made for this case.
The k-value of NO at Lyman alpha is 67.5 cm-1. This value might prove useful in atmospheric physics,
since one of the theories on the formation of the D layer is the photoionization of nitric oxide by radiation
in the region from 1100 to 1300 X, particularly by Lyman alpha.
5.4. PHOTOIONIZATION CROSS SECTION OF NO
As pointed out previously, the first ionization potential of nitric oxide has not been measured spectro-
scopically. Because of the complexity of the NO spectrum, a Rydberg series could not be found. There.
fore, an attempt was made to detect an ionization continuum in the region of the continuous absorption
(see Fig. 16).
The preliminary experiment was simple and was carried out with the experimental arrangement already
described (see Section 2.5). The cell for the ionization experiment was essentially an absorption cell pro.
vided with platinum stripe for the collection of electrons and ions. Only about 5 v between the platinum
plates were required to produce a saturation current, however, in most cases the voltage used was 10 v.
Currents measured were in the range from 10-1 to 10-10 amp and a commercial amplifier (Beckman micro-
microammeter, model RXG2) was used. The amplified current was fed into a Speedomax recorder. The
slit width of the monoehromator and other experimental conditions were identical with those used in the
absorption measurement. Blank rum showed that the photoelectric current due to reflected or scatteredlight was negligibly small. For this purpose measurements were made both with the cell empty and with •
the cell filled with gases with high ionization potential.
To obtain the ionization yield, it is necessary to know the number of photons absorbed per second
within the cell. The intensity of the light entering the cell was measured with a phosphor-coated photo-
multiplier that was previously calibrated (see Section 2.2). The measured transhaiosion of the LiF window
was used in this determination. Several pressures of NO ranging from 0. W ito 10.5 invui were j-,d, aradi the
fraction of the incident radiation absorbed in the cell was computed from the previously deterniaiurd A,- ahit.s.
These data were. adequate to give the ionization yield which it qlcdel l here as the ratio of Ilt ithe lmm.r-r of
ions formed per second and the total number of photons absorbed 1e, st-coatd.
Table 4 shows the yield obtained at a number of wavelengths. Thus, the ionization cross section of
NO in the region from 1060 to 1300 .. is of the order of 10-18 cml which is quite large. Since there are some
possibilities of systematic errors, the combined error might be large and is estimated to be about 50 percent.
A more accurate determination using direct energy measurements with a thermotouple would te dehirable,
The onset of ionization occurs at 1343 A L 2 A or 9.23 ev ± 0.02 ev. This appears to be- a more lore.
cise value of the first ionization potential of NO, than the value 9.4 ev ; 0.2 ev, obtained by ilagstrum
(1951) by the electron impact method. Recently, Miescher (1952) and Tanaka (.1953) independently
42
Table 4. iPhotoionizatiou 1 of NO.
() Yield ( Yield
1344 0.002 125 • 0.45
1342 0.010 12', .? 0.50
1340 0.029 1214 0.51
1338 0.080 12V' 0.52
I15,2 0.096 IM;5•. 0.49
1315 0.14 1, 0.
1292 0.35 1W80 0. 45
reported an emission band of NO+ ion with energy 9.05 ev. I? i', ri--.es~in0 '4j note that by &dding thiA
energy to the first ionization potential, one obtains the fourth ionization potential (18.28 ev) of NO found
by Tanaka (1942).
6. CARBON DIOXIDE
From the scarcity of discussion in the literature, it appears that carbon dioxid, does not play an impor-
tant role in the photochemistry of our atmosphere. Perhaps it is not safe to assume this, since so little is
known regarding the photochemistry of the atmosphere. Radiation in tht vacuum ultraviolet region
dissociates CO into CO and 0, and this process has been used as an actinometer for photochemical studies.
In this connection, it might he noted that the atmosphere of Venus is composed almost entirely of CO:and receives the same ultraviolet radiation; however, the expected photochemical product, CO, has not been
observed.
The absorption spectrum of CO2 in the vacuum ultraviolet has been studied by a number of investi-
gators including Lyman (1908), Leifson, (1926), Henning (1932), Rathenau (1934) and Price and Simpson(1938). Considerable attention has been focused on the electronic structure of the CO, molecule, especially
by Mulliken (1932, 1935, 1942) who made theoretical formulations for this molecule.
The absorption coefficients of CO2 in the region from 1440 to 1670 1• were measured by Wilkinson and
Johnston (1950). At lower wavelengths there appears to be only the measurement of Preston (1940) at
Lyman alpha.
The CO used in the present investigation was prepared in a vacuum purification system by repeated
sublimation of tank CO2, retaining only the middle fraction in each sublimation. Mass spectrometric anal-
ysis showed that the total amount of impurities in the purified sample was less than one part in 2000.
The measurement of t,.- k-values of COt required no modification in the experimental method. To
cover a wide range of absorption intensities, pressures of CO2 ranging from about 0.002 to 100 mm Hg were
used in the absorption cell. The pressure was varied by a factor of 10 to 50 for each wavelength, but the
k-values were found to be independent of pressure over the entire spectral range investigated. Furthermore,
43
20
Odt-15•L
C-
_ 00
z
z0
o, 5
1400 1450 1500 1550 I600 1650 1700 1750
WAVELENGTH IN ANGSTROMS
Fig. 17. The k-values of CO0 in the region from 1400 to 1750 A.
the data were obtained with two different gratings, one giving somewhat higher resolution than the other.
The k-values in the region from 1300 to 1400 X obtained with these gratings agreed within experimental
error, thus indicating that the resolution was adequate.
For convenience of discussion, the results are divided roughly into three spectral regions: the first region
covers the range from 1400 to 1800 A which consists of a continuum and a number of weak diffuse bands; the
second region, 1200 to 1400 A, likewise has a continuum and a number of diffuse bands; and the third region
at shorter wavelengths includes some very strong bands and at least one very intense absorption continuum.
6.1. RESULTS FOR THE SPECTRAL REGION FROM 1400 TO 1800
The k-values of %he absorption spectrum of COs in this spectral region are shown in Fig. 17 by the solid
curve. The results of Wilkinson and Johnston (1950) are shown by the dashed curve. Their curve should
be raised about 10 percent, however, since their absorption coefficients were defined for 300 C rather than
for 0* C. Even when this is done, their k-value at 1450 X is about a factor of 2 lower, and at 1660 . it
is a factor of 2 higher than our result. They found the maximum of the continuum at 1495 A; Price and
Simpson (1938) observed the maxiwum at about 1460 k. Since a number of diffuse bands are superimposed
on the continuum in an apparently irregular manner, the maximum of the continuum cannot be determined
precisely. By taking into account the fact that a second continuum (see Section 6.2) overlaps the first in
the region from 1400 to 1450 A, the maximum was estimated to be at about 1475 A. The result (Fig. 17)
agrees much better with the absorption spectrum of Price and Simpson than with the curve obtained by
Wilkinson and Johnston.
Using Eq. (1), the f-value of 0.0043 was obtained for the continuum (excluding the bands and sub-
tracting the contribution of the second continuum). T'his value is in good agreement with the value of
0.004 obtained by Wilkinson and Johnston (1950) in spite of the differences in the abhorption curves. Fuchs
(1927) has obtained a resonance wavelength of 1480 A for CO, from dispersion measurements. How-
ever, the corresponding f-value (0.075) is much higher than the values obtained by absorption measure-
ments, and Wilkinson and Johnston have pointed out the need for a revision in the dispersion equation.
The small f-value indicates a forbidden transition which Mulliken (1935, 1942) has already suggested:
IM+--+ (1.)-lo, 'h11,. He has also suggested the possibility that such a forbidden transition may occur
weakly if the upper state of the equilibrium form of the CO molecule is triangular.
The bands overlying the continuum have not been classified, and in fact some investigators have not
observed most of them. Leifson (1926) observed bands at 1692, 1675, 1660 and 1647 A, and Wilkinson and
Johnston (1950) observed bands at 1690, 1673 and 1662 A. On the other hand, Price and Simpson (1938)
did not list the wavelengths of the bands in this region, although their photograph seems to show most of
the bands observed in the present investigation. These bands appear irregular and diffuse, hence it is diffi-
cult to single out any vibrational progression. Most of them are listed in Table 5 along with the other ob-
served bands. In this table, the first column represents the approximate wavelength of the "middle" of the
band, the second column is the relative intensity of the band taking the strongest band at 1088 A as 1000.
Table S. Wavelengths (in A) of CO% absorption bands. Wavelengths given by Rathenau (1934). Price and Simlwon (1938).Leifion (1926) and Wilkinson and Johnston (1950) are indicated by their initial.
I by others A I by others X I A by others . I x by others
1072 70 1074(R) 1227 <1 1403 1 1549 <1
1081 50 1083(R) 1244 <1 1413 1 1555 I1088 1000 1089(R) 1252 <1 1418 <1 1570 <1
1093 20 1093(R) 1262 <1 1422 <1 1584 1
1101 20 1103(R) 1272 2 1426 1 1596 <1
1110 90 1112(R) 1284 2 1436 1 1605 <1
1119 800 1121(R),1120(P&S) 1293 5 1447 1 1613 <1
1129 400 1129(R),1130(P&S) 1303 5 1459 <1 1628 <1
1142 100 1142(R) 1316 3 1469 1 1645 <1 1647(L)
1150 20 1149(R) 1328 3 1478 <1 16,61 <1 1660(L),1662(W&J)
1158 30 1157(R) 1336 4 1483 1 1669 <1
1166 40 1165(R) 1341 2 1495 1 1675 <1 1675(L),1673(W&J)
1175 10 1174(R) 1349 3 1506 1 1686 <1
1183 60 1362 2 1517 1 1691 <1 1692(L),1690(W&J)
1193 <1 1371 1 1520 1 1695 <1
1203 <1 1381 1 1526 <1 1708 <1
1210 <1 1386 <1 1531 1 1715 <1
1217 <1 1394 <1 1543 1 1720 <1
-- -.. . - - .- --- -~-s- ta. ~ -. ~ '~. - -L
45
35
z 002S25-W20
o g0z
I.- 10-
0
1150 1200 1250 1300 1350 1400 1450
WAVELENGTH IN ANGSTROMS
Fig. 18. The k-values of CO1 in the region from 1150 to 1450 A.
and the last column gives the wavelength given by other observers. The table must be considered tenta.
tive, since further investigations are required.
6.2. RESULTS FOR THE SPECTRAL REGION FROM 1200 TO 1400 A
The second continuum ( at 1332 X) shown in Fig. 18 is a little more inwenm ithan the 1475 A con-
tinuum. The relative inteasities of these two continua do not seem to agree with the result of Price and
Simpson (1938). In their photograph of the absorption spectrum of CO2 at very low pressures ('-'0.01 ram),
the 1332 A continuum is not present while the 1475 & continuum appears. This may be explained by the
possible changes in the background intensity of the Lyman continuum.
"The f-value for the 1332 & continuum computed as in the previous case was 0.0053, which is slightly
larger than that of the first continuum. This low value may again be indicative of a forbidden transition to
a second excited electronic state.
The bands in this region are somewhat more intense and appear more regular than those in the first
region. Price and Simpson (1938) suggested that these bands may consist of two progressions starting
from 1380 A and 1368 A, respectively, each with a frequency interval of about 1225 cm-' corresponding to
the symmetrical vibration frequency of the ground state. An attempt to follow their suggestion was not
successful, however, since the shape of the bands and wavelength intervals is too irregular for wavelength
assignments.
46
10 0 0 0 _1 - r I- f I I I I l l ...5000 -
Co -C02
S1000
U. 5 00z
00 50-Z
a.cr 10--0
CD)"4 --
1050 1075 1100 1125 1150 1175 1200
WAVELENGTH IN ANGSTROMS
Fig. 19. The k-values of CO, in the region from 1050 to 12001.
The k-value at Lyman alpha which lies near the minimum on the short wavelength side of the continuum
was found to be 1.97 cn-1. This value agrees within experimental error with the carefully measured value(2.01 cm-1) of Preston (19140).
L.& RESULTS FOR THE SPECTRAL REGION FROM 1050 TO 1200 A
Evxtrere17 high absorption coefficients were obtained in this region as shown in Fig. 19 which is a semi.
log plot of the k-values. The highest k-value was about 4350 cm-1 at 1119 1.
The continuum with its maximum at about 1121 A is much more intense than the two continua at longerwavelengths. Thef-value computed for this continuum was 0.12 and this value indicates that the continuumis an allowed electronic transition.
The strong bands overlying the 1121 A continuum have been previously reported by Rathenau (1934)and Price and Simpson (1938). The strong bands at 1119 and 1129 X have been shown by the latter tocontain the second members of a Rydberg series converging to the first ionization potential at 13.73 ev.The other bands have not been classified. Our data, however, seem to show a progression with a frequencyinterval of about 630 cm-' which would correspond to the bending vibration in the ground state.
Below 1100 A there appears to be a portion of another strong continuum. However, the data obtainedin this region are somewhat poor because of absorption of the LiF windows of the absorption cell, and dataobtained in the region from 1050 to 1080 A were rejected.
47
0
3.3 3 o4u
6.5 (4.4) - 2 7rLu2. 0. d edW 4oeuafk
Urmhia f* COS.
8.0 (6.0) - 3 0-g
13.5 (13.3) -lrg
Xmax 1335 1600
64. DISCUSSION ON ELECTRONIC TRANSITIONS
It is possible to make some comparisons of our results with certain theoretically predicted electronictransitions suggested by Mulliken (1942). A portion of one of the energy diagrams used in his paper isreproduced in Fig. 20, showing only the outermost occupied level of COs and three unoccupied ones. Thecolumn on the right designates the molecular orbital and the column on the left the ionization energy inelectron volts (averaged for C%) and CS,) corresponding to each orbital. The values in parentheses are theionization energies for the C02 molecules which Muiliken gives for the vwriaa processes corresponding to
Table 6. Dissociation energies of COs in eectron volts,
Experimental ValuesStates Calculated Values (Upper Limit)
CO(Z) + 0(P) 5.4 6.8
CO(Z) + OQD) 7.4 7.7
CO(QZ) + O(S) 9.7 10.3
the absorption maxima at 1600 and 1335 A (from the data of Price and Simpson). Instead of 1600 A if thevalue 1475 1 or 8.4 ev, corresponding to the maximum of the first continuum, is used for the lr, --+ 3e,transition, we obtain the ionization energy
13.7 - 8.4 - 5.3 ev
for the 3a, orbital. For the lr, --+ 2 1r transition our results are substantially the same as Mulliken's, since
the wavelengths used for the second maximum are about the same (1332 and 1335 1). The absorptionmaximum at 1121 A (11.1 ev) seems to correspond to the transition hr, -4 3a. energetically. This would
give for the ionization energy of the 3a. orbital a value of about 13.7 - 11.1 = 2.6 ev.
The dissociation energy can be measured from the long wavelength limit of a continuum. Lai practice,this is often neither possible nor reliable. Even when the case is not an ideal one, a good estimate of the
upper limit of the dissociation energy can be obtained, provided that the extrapolation is not too extensive.Using this method, dissociation energies (upper limits) of 6.8, 7.7 and 10.3 ev were obtained from the threecontinua. These may be compared with the values listed by Weeks (1949) as shown in Table 6.
The values for the lowest dissociation energy do not agree too well. This may be explained by the po"a-bility that ihe absorption continuum arise from a transition to a relatively steep repulsive curve for the
dissociation products CO('Z) + O('P). In fact, Weeks shows such a curve for this dissociation. Thetwo higher dissociation energies are in good agreement. considering the fact that the experimental values
are upper limits.
7. NITROUS OXIDE
The presence of nitrous oxide in our atmosphere is now well established. The first observation was
made by Adel (1939) who attributed an atmospheric band at 7.8 j to N20, and since then his observation
was confirmed by a nunmber of investigators who also observed other infrared bands of N2O in the solarspectrum. The total amount of N2O in the atmosphere corresponds to about a layer thickness of 4 mmreduced to NTP according to McMath et al. (1950). The origin, amount and distribution are not suffi.ciently well known to establish the importance of NO in the photochemistry of our atmosphere. Thepossibility of the formation of NO from NgO was pointed out at the beginning of Section 6.
A few investigators have studied the absorption spectrum of N20 in the vacuum ultraviolet. Leifson(1926) observed two continuous bands, the first covering the region from 2000 to 1680 A and the second
extending from 1550 A to the transmission limit of fluorite. However, he did not observe any discrete
-~ -- -- -- -- - -- - - ýw -Lx --;C1 -- ' -.-
49
1lO00 N20
I--000.
WULA.LA.
1I00 -
1080 1100 1120 1140 1160 1180 1200 1220
WAVELENGTH (A)
Fig. 21. The k.valucs of NO in the region from 1080 to 1220 A.
bands. Sen-Gupta (1935) also observed two continua in about the same regions. The most thorough anddetailed work is that of Duncan (1936) who studied the region from 850 to 2200 A On the other hand, the
near-ultraviolet absorption spectrum of N20 has been studied by a number of investigators.
The absorption coefficients of NtO in the ,eegion from 1390 to 2200 X were recently measured by Romandand Mayence (1949). They measured the k-values of the two continua mentioned above and found maximaat 1450 and 1840 A. There appear to be no other measurements of k-values in the vacuum ultraviolet.
In the present investigation, the N20 was purified from tank gas by drying over P20 and ten successivefractional distillations with liquid nitrogen, retaining the middle fraction each time. The final product wasanalyzed with a mass spectrometer. No detectable impurities were found, indicating that the total im-
purities could not exceed 0.05 percent.
Absorption coefficients were measured from 1080 to 2100 A using pressures from about 0.01 to 200 mmHg, to cover the absorption intensities. Beer's law was found to hold over the entire spectral range, withinexperimental error of 5 to 10 percent.
For convenience of discussion, the absorption spectrum of N20 will be divided into four regions, eachof which will be designated by the terminology of Duncan (1936). Pegion E-F will cover the range from1080 to 1215 A, region D from 1215 to 1380 A, region C from 1380 to 1600A and region B from 1600 to 2100 A.
7.1. RESULTS FOR REGION E-F
Figure 21 is a semilog plot of k vs A from 1080 to 1215 A. Wavelengths below this region could not be
studied in this work, since the transparency of LiF extends only to about 1050 A. Rather diffuse bands
50
3000
N20
o 2 40 0
z
"--U.
12002
6W-
Ca00J
1200 1230 1260 1290 1320 1350 130
WAVELENGTH (A)Fig. 22. The k.valueu of NO in the region from 1220 to 1380 A.
were found at wave numbers 85,350; 85,800; 86,200; 86,7001 87,500; 88,50M, 89,450% 90,400; 91,000 and 91,800cm-'. Nearly all of these bands appear in the photograph of Duncan's paper. The wave numbers of thebands agree quite well with those Duncan has given except for the bands at 88,500 and at 91,000 cm-'. Forthe latter, he gives the value 91,200 cm-1. According to Duncan, this band is a member (n = 4) of aRydberg series represented by
P a- 102,567 - ( R where n - 3, 4, 5 .... (2)(n - 0.92)1
The calculated wave number for n - 4 is 90,999 cm-1, in closer agreement with our value of 91,000 cm-1.It must he admitted, however, that the wave numbers of the bands in this spectral region are only approxi-mate values, owing to the diffuse nature of the bands. Therefore, it was not possible to classify them. Theymay he due to one or more electronic transitions.
In thiij region there is another band at 84,900 cm-1 which is the strongest ultraviolet absorption bandobserved for NfO. Its k-value at the maximum is 3010 cm-1. Duncan (1936) suggested that it is a mem-ber of a Rydberg series converging to an ionization potential at a wavelength below our limit of observation.
51
250
tzoo- NP0 8.-
1150- 6D
8 "00 4j.
5 0o 2.0.
1360 1410 1440 1470 1500 1530 1560 1590 1620
WAVELENGTH (A)
Fig. 23. The k-values of NO in the region from 1380 to 1600 M.
Most of the absorption intensity in this spectral region may be attributed to a continuum which appears
to underlie the diffuse bands. The maximum of the continuum cannot be established from Fig. 21. Flow.ever, Duncan's photograph shows a relatively transpirent region at about 1060 A. and it appears that the
absorption intensity of the continuum decreases rapidly from 1080 to 1060 A. Thus, the shape of the absorp-
tion curve for the continuum is inferred to be quite asymmetric. The maximum of the curve is probably
near 1080 A. On this basis, the f-value of the continuum (excluding the bands) was estimated to be about
0.1 (see Eq. (1)) which indicates an allowed transition.
7.2. RESULTS FOR REGION D
Figure 22 is a linear plot of k vs X from 1215 to 1380 A. Whereas Duncan has reported this region to
contain a perfectly smooth continuum having no structure, the present study has located several weak
diffuse bands on its long wavelength slope and one sharp band at 77,400 cm-' which may be a member of the
Rydberg series described by Eq. (2). Duncan chose the maximum absorption of the continuum at 77,900cm-1 as his Rydberg member (n = 3). The band at 77,400 cm-1 fits the Rydberg series much more closely
than does the continuum maximum at 77,900 cm-1, the calculated value being 77,202 cm-n. It is possible
that this band is very strong and that the continuum is actually somewhat weaker, with its maximum value
appearing to be high because of poor resolution of the sharp band. The absorption of the Rydberg bandprobably contributes to the maximum value of the continuum (k - 2465 cm-'). Diffuse bands are found
at 77,100; 76,250; 75,600 and 75,100 cm-1. The wave numbers given here are probably not very accurate,
52
Table 7. Bands of electronic transition C of NAO.
A Calculated Olserved AV Relative Intensity
0 59,590 59,520 70 0.X)
1 60,565 60,510 55 0.08
2 61,484 61,580 -96 0.18
3 62,349 62,.!1) - 111 0.20
4 63,164 63,250 -86 o.68
5 63,931 64,020 -89 1.68
6 64.654 64.720 -66 1.60
7 65,337 65,360 -23 5.0
8 65,981 65,980 1 8.0
9 66,591 66,580 11 24
10 67,173 67,160 13 43
II 67,720 67,7(0) 20 70
12 68,246 68,210 36 102
13 68,749 68,730 19 102
14 69,234 69,230 4 69
]5 69,703 69,710 -7 44
16 70,160 70,180 -20 17
17 70,609 70,650 -41 1
18 71,050 71,100 -50 1
19 71,490 71.530 -40 1
20 71,930 71,940 -10 1
owing to the diffuse nature of thene bands. It is possible that region D consists of more than one electronictransition with the upper state of one being repulsive. In addition, the sharp, strong band is tentativelytaken as a Rydberg series member. Neglecting the Rydberg band at, 'the possibility that the continuum maybe somewhat weaker than it appears to be, thef-value of the continuum is calculated to be 0.367. The limitsof integration are a - 82,250 cm-1 and v = 73,000 em'. The high f-value suggests an allowed transition.
7.3. RESULTS FOR REGION C
Figure 23 is a linear plot of k vs A for this region. This region is composed of a continuum on whichabout 20 diffuse bands are superimposed. Duncan (1936) reported nine bands which fit the formula
Sa 65,939 + 621.2 n - 11.54 n' (n = 0,1,2... ). (3)
He was not able to relate the frequency 621 cm-1 to the fundamentals (589, 1285 cm-) observed in theinfrared, and suggested that the value he obtained was probably a modification of 589. Sponer and Teller(1941) suggested two other possibilities: (1) that the observed bands may represent two systems, each with621 X 2 cm-1 spacing; and (2) that the bands observed by Duncan were possibly not the lowest members of
53
NO02Z -
LL-
z0
1600 162 1650 1675 1700 800 1900 2000WAVELENGTH (A)
Fig. 24. The k-value. of NO in the region from 1600 to 2100,L
the progression, in which case there might be considerable anharmonicity leading to a lowering of 1285 em-'.
The present work has detected many more bands in this region, both at the shorter and longer wave-length sides of the region where Duncan found 9 bands. These bands are listed in Table 7 and theyapproximately fit the equation,
P - 59,590 + 1005 n - 30.0 n' + 0.53 ns, where n - 0, 1, 2 .... (4)
Some of tePe bands which fit Eq. (4) are in region B.
The value 1005 cm-1 seems to be consistent with suggestiojns (2) above by Sponer and Teller. Thediscrepancies between calculated and observed frequencies are not surprising, in view of the diffuse natureof these bands. The strongest absorption lies at the band 68,730 cm-1.
There appears to be a relatively strong continuum underlying these bands with a maximum absorption- coefficient of 147 cm-1 at 68,940. This value compares fairly well with that of Romand and Mayence (1949)
who found the k-value 118 cm-'. The discrepancy might he attributed to the difference between photo-graphic and photoelectric detection. The continuum is quite symmetrical and both ends can he extra-polated to sero without much difficulty. The f-value was measured as 0.0211, suggesting a fairly allowabletransition. The limits of integration werev -- 73,000 cm-n and v - 64,250 cm-x.
7.4. RESULTS FOR REGION B
Figure 24 is a linear plot of h vs X for this region. This region is one of weak continuous absorptionwith a maximum k-value of 3.82 cm- 1 at 1820 A. Romand and Mayence found a maximum at 1840 X withk - 3.13 cm-'. Here, as in the preceding section, the discrepancy is probably due to the difference in methodof detection. Once again, superimposed on this continuum are weak, very diffuse bands. Because oftheir diffuse nature, the wavelengths could not he measured accurately, and no attempt was made to classify
them. The f-value (0.0015) of the continlutn wais mcasured as in the prc-ious section with integration
limits Y = 65,000 cu-f and ( = 46,000 cm'-. The limits are probeal)y not accurate, particularly the long.
wave limit. Various authors have found different results for this value. The weak ,haracter of the absorp.
tion suggests that the transition is forbidden. It is difficult to say whether there are one or two electronic
transitions involved here, although there seem to be two, with the uppe:r state of one being repulsive and
the upper state of the other having a nlinimum.
8. NITROGEN AND IIYDROGEN
Very few absorption measurements of nitrogen and hydrogen in the Schumann region have been re-
ported in the literature. This is due to the fact that both gases are relatively transparent in the spectral
region above about 1000 A. On the other hand, many investigators have studied these molecules, using
their emission spectra both in the vacuum ultraviolet and in the longer wavelength region. Most of their
results on the energy states of these molecules are summarized in the book by Herzberg (1950).
In the present work, it was not possible to obtain quantitative k-values for both gases, since the equip-
ment used was not designed to resolve the fine structures of the observed bands. The small amount of
data, however, may provide some idea of the order of magnitude of the absorption intensities of the two gases.
8.1. NITROGEN
The so-called Lyman-Birge-Hopfield bands (a forbidden transition, X '1+ -, a 'IIi) were observed by
Birge and Hopfield (1928) in absorption. They used an absorption path length of 100 cm and a nitrogen
pressure of 300 mm Hg. The ten bands, 0,0 to 9,0, which appear in their photograph are narrow, sharp
bands. About the same time, Sponer (1927) also observed these bands using a path length of 100 cm and
a pressure of 100 mm Hg. It is to be noted that Sponer did not find appreciable absorption by nitrogenwith a 10-cm absorption cell filled with N2 at a pressure of 800 mm. Thus, it appears that a much larger
product of path length and pressure is required to find new absorption bands in this region.
Recently, Clark (1952) reported k-values of N2 at 27 wavelengths in the region from 584 to 1026 A,and Weissler, Lee and Mohr (1952) reported k-values at 88 wavelengths in the region from 303 to 1306 A.Since different light sources were used, comparison can be made only at two wavelengths (1025.7 1 and
584.3 1). The former obtained 8 and 480 cm-' while the latter obtained 60 and 700 cm-1, respectively, at
the two wavelengths.
The data by Weissler, Lee and Mohr give k-values at 11 wavelengths in the region above 1100 A, the
values ranging from about 5 to 160 cm-1. Most of the values are for wavelengths lying between the heads
of the Lyman-Birge-Hopfield bands. For example, they obtained a k-value of 66 cm-1 at 1215.129 A,which lies between the 8,0 band (1226.6 A) and the 9,0 band (1F,',.3 A). This would indicate that there
are absorption lines in the regions which have been considere4 yye transparent. Preston (1940) fL.ind
55
Table 8. The apparent k-value. of the Lyman.Birge.Hopfield bands.
Band k Band k Band k
0,0 0.08 4,0 0.09 8,0 0.02
1,0 0.09 5,0 0.08 9,0 0.02
2,0 0.11 6,0 0.09 10,0 0.02
3,0 0.07 7,0 0.03 11,0 0.02
that the k-value of No at Lyman alpha (1215.6 1) is less than 0.005 cm-1. Since nitrogen is a major constit-uent of our atmosphere, much more data on its absorption intensity seem to be required.
In the present experiment, the cell length was 4.7 cm and the highest pressure of No was about 500 mm
Hg, the product being about one-tenth that used by Birge and Hopfield (1928). At this pressure, a weakbut measurable absorption of the Lyman.Birge-Hopfield bands from 0,0 to 11,0 was observed. The bands
appeared narrow (5 A or less) and no other absorption was detected between the bands. This was con-sistent with the absorption spectrum obtained by Birge and Hopfield (1928) and, furthermore, the data
gave no indication of impurities. At Lyman alpha the k-value was found to be definitely less than 0.01cm-1 in agreement with Preston (1940).
The apparent P-values of the peaks of the Lyman-Birge-Hopfield bands are listed in Table 8. As was
pointed out, these k-values are certainly not quantitative. Considering the type of data, the intensity dis-tribution is roughly in agreement with the photograph of Birge and Hopfield. Table 8 shows an ablupt
change in the band intensities going from the 6,0 band to the 7,0 band, the energy of the wavelength regionbetween these two bands being about 9.8 ev. It is interesting to note that predissociation at about this
level has been observed by Herman (1943, 1945) and by Herzberg and Herzberg (1947) in emission spectra.
One result which can be stated with some confidence is that no continuous absorption in the region
from 1600 to 1050 A with k-value greater than 0.01 cm-1 was observed for N,.
8.2. HYDROGEN
Still less can be said about the absorption of H2 in the same spectral region. Dieke and Hopfield (1927)
and Tanaka (1944) studied the absorption spectrum of H2 which extends into this region. The first four
members of the Lyman bands appear in the region from 1050 to 1110 X. These bands were observed at a
pressure of about 50 mm Hg in our absorption cell. Hence, their absorption intensities appear to be about
ten times greater than those of the first seven members of the Lyman-Birge-Hopfield bands of No,
Incidentally, the absorption data for Ht showed some absorption bands of H1O. Using the k-values
of H20 the amount of this impurity waa found to be about one part in 101 which was just beyond the sensi.
tivity limit of the mass spectrometer. This shows that in certain cases it is possible to detect quantitatively
small amounts of impurity by absorption measurements in the vacuum ultraviolet. For example, one part
of H20 in 100 parts of rare gas or other transparent gas can be detected with a slight modification in the
system used.
9. CARBON MONOXIDE*
In emission spectra, the carbon monoxide molecule is one of the most thoroughly studied diatomicmolecules. Very few investigations have been made in absorption, however, because of the fact that itsabsorption spectrum lies mainly in the vacuum ultraviolet. Further investigation of the absorption spectrumof CO is desirable to determine its highly excited states, ionization potentials and dismciation energies thatare uncertain or unknown. For example, a number of values have been proposed for the dissociation energyof CO, mostly from the study of predissociation in emission spectra, but the correct value is not known withcertainty. This value would be very useful in the study of other molecules with the carbon-oxygen bond.
Unlike N, and Hs, CO has a rich absorption spectrum in the spectral region from 1100 to 1600 1. Thereare many strong bands, particularly the well-known IVth positive bands. Because of their sharpness,however, very little quantitative data were obtained in the present absorption measurements.
The CO sample for the absorption measurement was prepared by dropping formic acid into concen.trated sulfuric acid. The sample was analyzed with a mass spectrometer and found to contain less than0.1-percent impurity. Pressures of CO from 2 to 300 mm Hg were used in the absorption cell.
For convenience of discussion, the result will be considered in terms of two spectral regions. The dataare summarized in Fig. 25 which covers the region from 1060 to 1600 A. The ordinate is labeled "absorp-tion intensity," since the k-values were apparent ones and were obtained with insufficient resolution.
9.1. RESULTS FOR THE SPECTRAL REGION FROM 1170 TO 1600 A
There were a number of sharp bands predominating this region. Most of them belonged to the fourthpositive bands (X 1Z - A4 A11). The members of the V.progression (with v" - 0) from v' - 0 to i - 17
were observed. As shown in Fig. 25, the first five bands of this progression are about equally intense inabsorption, and they are the strongest bands in the entire spectral region. The 5,0 band is much weakerthan the 4,0 band. Between the 13,0 and the 14,0 bands, there is another abrupt change in intensity. It isinteresting to note that these intensity anomalies correspond to two of the bands in which predissociationswere observed. Namely, Schmid and Gero (1938) have observed predissociation phenomena at t/ - 4, 9and 13 in the upper state of the fourth positive bands. Using emission spectra, Schmid and Ger6 statedthat the last vibrational level of the A 'IH state must he v' = 13, since they did not observe emission bandsoriginating from iv > 13. In the present work, the bands were observed up to v' = 17; however, bands withV > 13 were very weak.
There appears to be a weak continuum in the region from 1300 to 1600 A underlying the fourth positivebands. This continuum resembles the Schumann-Runge continuum. If this is the case, the amount of02 computed from its k-value is about one part in a thousand. Although the spectrum shown in Fig. 25is quite complex in the region below 1300 A, there are a few bands that might be due to oxygen. Theirintensities indicate, however, that the amount of 02 is somewhat less than the above estimate. Therefore,the presence of 02 in the sample seems probable, and the amount is somewhat less than indicated by the
* Dr. Y. Tanaka analywd the data fa, this ga.
57
1 In"positive bonds (vul 0, vl)-progression~~~~iI I, I~;,2 I I I ' I , I I I I , I ,, I I? v
C,)zWI-z
1.0z00 i
0
~0.01l._J.-..•_•.L
1100 1200 1300 1400 1500 1600
WAVELENGTH (A)
Fig. 25. Absorption curve of CO in the region from -1050 to 1650 X.
apparent continuum in the region from 1300 to 1600 A which appears high because of the contributionfrom the hands.
9.2. RESULTS FOR THE SPECTRAL REGION FROM 1060 TO 1170 A
Several strong absorption bands were observed in this region. Most of them have rather symmetricintensity distribution, and their wavelengths agree well with those of the X --# C and X -- E transitionswhich were found by Hopfield and Birge (1927).
In this region, there appears to be a continuum beginning at about 1140 A and its intensity is moder.ately strong at 1060 A. It may be one of the dissociation continua of CO, ,ond if so, the value 10.9 ev (or1140 A) would correspond to the upper limit of the dissociation energy of the ground state, X 'z+. Thisvalue is very close to the dissociation energy of the X 'I+ state (10.11 ev) proposed by Gaydon (1950),but the present data is not adequate to permit any conclusion.
In general, the data obtained for CO was unsatisfactory, and future measurements with higher resolu.tion are considered.
58
1000
I-.ZW 100
W0U- i0
Z ~NH 3
a-m~a:
C,)
I I i I I1600 1700 1800 1900 2000 2100
WAVELENGTH (A)
Fig. 26. The k-value, of Nil3 in the region from 1600 to 2200 ,.
10. AM•MONIA
The absorption spectrum of ammonia in the vacuum ultraviolet was most thoroughly studied by Duncan(1935, 1936 a). He confirmed the •arlier observations of Leifson (1926) and Dixon (1933) in tile longerwavelength region and extended their work down to 850 A. The rotational structure of seven bands inthe region from 1450 to 1620 , belonging to the second excited state above normal was anal. zed by Duncanand Harrison (1936) with a 21-ft focus vacuum spectrograph. There appear to be no k values in the vacuumultraviolet for NH3 reported in the literature.
The ammonia gas was prepared from tank NH3 by repeated fractional distillatiorls, retaining only themiddle fraction. Mass spectrometric analysis showed that the amount of impurities was less than the limitof detection (i.e., less than 0.05 percent).
Pressures of NH3 used in the absorption cell were from about 0.02 mm to 15-mm fig and were measuredwith two McLeod gauges. Although the compressibility of ammonia is comparativelv high, the error frontthis cause was computed to be less than one percent. (See Farkas and Melville (1939).) The adsorptionof NH, on the wall was more troublesome. There were some pressure variations during the absorptionmeasurements, but, covering a limited range of spectrum at a time, the error from thbi, source was keptbelow five percent. Adsorption of NHs on the LiF windows was detected by measuring thie light intensityafter the NH, was pumped out of the absorption cell, but the absorption due to the adsorbed NH3 was very
weak. As a rule, the k-values obtained for different pressures were consistent.
59
1000- I I
--SC ,
zC-,
LL
0 NH3
z0 10a.:0
C,)
i I I I I1100 1200 1300 1400 1500 1600
WAVELENGTH (A)
Fig. 27. The k-vahles of Nil, in the region from 1050 to 1650 k•
10.1. ABSORPTION COEFFICIENTS AND CONTINUA
Figures 26 and 27 show the k-values of Ni13 in the spectral region from 1050 to 2200 A. The values are
quite high throughout the spectral region, so that it was not necessary to use pressures of NH 3 higher than
about 15 mm ig. The highest k-value in this region was 980 cm-1 at about 1342 A. The k-value at Lyman
alpha was 190 cm-1. The bands in the region from 1300 to 1600 A showed some apparent pressure effect.
There appear to be at least three continua underlying the numerous bands which extend over the entire
spectral region. The first continuum is a rather broad and symmetric one which can be seen readily in
Fig. 26 extending from about 1600 to 2200 k. Thef.value for this continuum with a maximum at about
1870 A can be computed from the data by using Eq. (1), since the extrapolation of the wings of the continuum
was comparatively simple. Thef-value was found to be 0.030 (not including the bands).
The region below about 1550 A (Fig. 27) is somewhat more complicated. A second continuum seems
to cover the region from 1550 to 1200 A with a maximum at about 1315 A. The lower wavelength limit of
this continuum is indefinite, since it overlaps the third continuum, and the f-value of the second continuum
was estimated to be about 0.02.
The third continuum has a maximum at about 1100 A. The present measurement did not reach far
enough to the shorter wavelength end of this continuum, so the f-value cannot be determined accurately.
A rough estimate would be about 0.1. The possibility that this continuum may be partly or largely due
to photoionization will be discussed in Section 10.3.
60
10.2. BAND SYSTEMS
A comparison of the bands in Figs. 26 and 27 with those observed by Duncan (1935, 1936 a) shows goodagreement with respect to the wavelengths and the relative sharpness or diffuseness of the bands. Duncanhas already classified the bands into four different progressions, which he labeled Series I to IV, and hasobtained series formulas for each progression. Duncan (1935) lists 15 bands in Series 1 (2170 to 1680 A),12 bands in Series 11 (1670 to 1390 1), 13 bands in Series III (1440 to 1220 A) and 4 bands in Series IV(1210 to 1160 1). Nearly all of these bands were observed; however, the assignment of these bands to thefour series, in a few instances, is not unambiguous in the present investigation.
According to Duncan, the frequency in all four v'.progressions involves only the v2 frequency and allfour excited electronic states are of the same type as the ground state. The latter state has been designa:edby Mulliken (1935 a) as 'Ap. Duncan (1936 a) encountered a difficulty regarding the assignment of seriesII, and tentatively assigned it as a 'A, state and the three other electronic states as 'A, states.
Duncan (1935) observed that the maximum of intensity in all four series appeared at about " = 7-9.Tiais indicated that the "tr" for the excited states are the same, although all are displaced from the "r." of thenormal state by a considerablo amount. From the data shown in Figs. 26 and 27, the maximum intensityseems to be at v' equal about 5 (using Duncan's assignment of V') for series I through III. In other words,the change in "r." for the excited states with respect to that of the ground state appears to be somewhatless than the result of Duncan. There appears to be an intensity anomaly between v' = 6 and t f- 7 inseries II (see Fig. 27); however, this may be due to the separation of a secondary band head in Vi = 7.
The bands in series I are quite broad and diffuse as compared with the bands in series II, II1 and IV.The diffuseness has been ascribed to predissociation by Bonhoeffer and Farkas (1927). The three bands(0,0, 1,0 and 2,0) at the long wavelength end of series I (see Fig. 27) are double-headed as found by Dixon(1933) and Duncan (1935), and the separation of the bandheads (,'70 cm-1) are also in agreement.
10.3. IONIZATION POTENTIAL OF NH,
Using the four excited electronic states, Duncan (1936 a) attempted to obtain a Rydberg series converg-ing to the first ionization potential of NH,. He found, however, that the 0,0 bands of the four series did notfit a Rydberg series. By omitting series II and using the wave numbers of the most intense band of theremaining series, he obtained a formula converging to a limit at 11.5 ev. This value apparently agreeswith the values obtained by the electron impact method; Mackay (1924), Waldie (1925) and Bartlett (1929)obtained 11.1, 11 and 11.2 * 1.5 ev, respectively. Nevertheless, the formula obtained by Duncan appearsto be unsatisfactory, since he omitted series II.
In order to determine the ionization potential, the method applied to NO (see Section 5.4) was usedfor NH,. Using this method of measuring photoionization, the onset of ionization was found to be at10.12 +0.05 ev. This value agrees quite well with the most recent electron impact value obtained byMann, Hustrulid and Tate (1940) of 10.5 ev +0.1. Furthermore, the 0,0 member of series IV by Duncanlies at a wavelength (1207 A) shorter than that for our value of the ionization potential (1225 1). It ispossible that series IV is part of series III and members beyond ef - 12 are preionised, thus appearing as adifferent series.
61
The continuum underlying the hands in tile region below about 1225 A must be due partly to photo.
ionization. Since experiments on the ionization cross section have niot been completed, the contribution of
photoionization is not known.
A word may he added to tie measurement of tile ionization potential by the method used here. In the
case of ethylene, the ionization potential by this method was found to be 1.0.46 ev. This is in agreement
with the value of 10.45 ±0.03 from vydberg series obtained b) Price and Tutte (1940).
11. WATER VAPOR
At present, vern little is known regarding the importance of water vapor in the region above the trop-
osphere. The presence of this gas at high altitudes, up to at least 80 or 90 kin, is known from observations
of noctilucent clouds. The recent identification of the Oil radical ip the emission spectrum of the night
sky by Meinel (1950) has aroused considerable interest in 1120 as a constituent in the high atmosphere.
Bates and Nicolet (1950) made a theoretical study of the photochenmistry of atmospheric water vapor for
the altitude range from 60 to 100 kin.
The absorption spectrum of H20 vapor in the vacuum ultraviolet has been studied by several investi-
gators. The early work of Leifson (1926) was extended by Rathenau (1933) and Henning (1932) down to
shorter wavelengths. Later, Price (1936) observed two sets of Rydberg series in the region from 1000 to
1250 A. One of these yielded an accurate value of the first ionization potential (12.56 cv) of water vapor.
Recently, Hopfield (1950) pointed out that water vapor may be important in the absorption of solar
radiation, particularly in the region of Lyman alpha, and stressed the need for measurements of absorption
coefficients. About the same time, Wilkinson and Johlston (1950) reported k-values of 1120 in the region
from 1450 to 1850 A. Below this region there appear to be no data except the measurement of Preston
(1940) at 1215.6 A.
The water vapor used in the present investigation was obtained from distilled water after repeated
fractional distillation in a vacuum purification system at 00 C, retaining the middle fraction in each distilla-
tion. The fractional distillation was slow at this temperature but more effective in the removal of impurities
than a similar process at a lower temperature. Mass spectrometric analysis of the water vapor showed no
detectable amount of impurities.
Pressures of water vapor from 0.08 to 8 mm were used in the absorption cell. These pressures were
measured with an alphatron gauge (National Research Corporation) which was calibrated against two
McLeod gauges. Since the alphatron gauge reads the instantaneous pressure, it was particularly convenient
for H20 vapor which adsorbs strongly on the walls of the glass system. The water vapor was contained
in a liter flask at a pressure somewhat below its vapor pressure at room temperature and released into the
absorption cell whenever required. The adsorption of water vapor was so serious in some cases that it was
necessary to allow the vapor to remain in the absorption cell for about an hour before absorption measure-
ments were made. In spite of the precautions taken, the pressure during a run varied up to 10 percent and
corrections were applied to the data.
62
500 n 1 1 1 1 1 1 1 , 1
S 200 H20
100
z
50--iU.2
10-S 5--
0
1300 1400 500 1600 1700 800
WAVELENGTH IN ANGSTROMS
Fig. 28. The k-values of H2O in th, region from 1250 to 1850 1
11.1. RESULTS FOR THE SPECTRAL REGION FROM 1450 TO 1800 X
The k-values of water vapor in this spectral region are shown in Fig. 28. The absorption here is almostentirely due to a continuum with a maximum at about 1655 X. There are some indications of very weakbands in this region; however, the variations in the intensity are less than S percent, which is approximatelythe experimental error, so the presence of these bands is not certain. Rathenau (1933) reported seven weakbands in this region but the wavelength of most of these bands cannot be fitted with the "indications"mentioned above.
The k-value at the maximum of the continuum was 124 cm-1. This value is about 10 percent higherthan the value reported by Wilkinson and Johnston (1950). The k-values over the continuum are in fairagreement with their result; however, the three bands they reported were not found. Since the k-valuesof the minima between the peaks of the three bands are about 20 percent lower thin the maximum k-value,these bands should be readily detected by the present method which appears to have better resolution andsensitivity to small variations in absorption intensity. The f-value for this continuum was found to be0.041 as compared with 0.03 by Wilkinson and Johnston.
63
11.2. RESULTS FOR SPECTRAL REGION FROM 1250 TO 1450 A
As shown in Fig. 28, a number of diffuse bands were found superimposed on a second continuum. Thewavelengths of these bands are listed in Table 9, together with the bands observed by Rathenau (1933).The latter gave the wavelength range of the bands and not the wavelength of the maximum of each band.By using the wavelengths of the band centers, however, he obtained an interval of about 800 cm-1- Theresults shown in Table 9 are in good agreement, considering the fact that these bands are diffuse and itheheads of the bands cannot be measured accurately.
Table 9. Wavelengths (A) of the diffuse bands ofHsO vapor in the region from 1250 to 1450 A.
Rathenau Preeent Observation
1411
1392
1378
1361-1372 1364
1346-1357 1348
1332-1341 1335
1318-1324 1321
1306-1309 1308
1291-1297 1295
1279-1284 1281
1267-1271 1269
1253-1257 1257
The shorter wavelength limit of tile continuum underlying the diffuse bands was estimated to be about1150 1. On this basis, the f-value of the continuum was found to be about 0.05.
11.3. RESULTS FOR THE SPECTRAL REGION FROM 1050 TO 1250 A
The k-values of HtO vapor in this spectral region are shown in Fig. 29. Here there are a number ofstrong bands, some of which are diffuse and others appear to have structure. Lyman alpha lies near thepeak of one of the broad diffuse bands, and its k-value was found to be 387 cm-1. This value is in excellentagreement with the value (390 cm-1) obtained by Preston (1940).
64
T H20XUz
i _10-
S100 n go
1100 1150 1200 1250
WAVELENGTH IN ANGSTROMS
Fig. 29. The k-values of 11,0 in the region from 1050 to 1250 A.
Both Rathenau (1933) and Price (1936) studied this region. The latter, using higher dispersion, wasable to resolve a great deal of the rotational structure but found it too complicated for complete analysis.In the present investigation, the k-values for the bands in the region below 1150 A showed some "apparentpressure effect" which suggested that the bands were somewhat sharp and that the resolution used was notsufficient. However, the k-values of the bands in the region from 1150 to 1250 A showed very little changewith pressure and, hence, should be fairly reliable. A number of bands in this region were identified byPrice (1936) as members of two Rydberg series, These bands appear in Fig. 29. The higher members ofthe series were not observed, since they lie beyond the transmission limit of LiF.
12. METHANE
The vacuum ultraviolet absorption spectrum of methane has been studied by several investigators.Leifson (1926) studied the region from 1215 to 1800 A and found the absorption to be everywhere continu-ous, except for six bands between 1558 and 1420 X. Rose (1933) was unable to find these bands and sug-gested that they appeared as a result of gaps in the band spectrum of hydrogen which Leifson used as back-ground source. Duncan and Howe (1934), using as source a condensed discharge in hydrogen, which pro-duces a true continuum, studied the methane spectrum down to 850 . and concluded that the absorption
63
1000-
0
z
zW_ _CHU.iIL
00
I-.0.0U)
I I 1 I1100 1200 300 1400 1500 I600
WAVELENGTH (A)
Fig. 30. The k-values of CIl 4 in the region from 1050 to 1600 A.
was continuous over the entire region. Absorption coefficients of methane in the region from 1370 to 1455 Awere measured by Wilkinson and Johnston (1950) who found a continuous rise of absorption intensity withdecreasing wavelength. The present work deals with the absorption coefficients of methane between 1065and 1610 1.
The methane was purified from Matheson tank gas by repeated fractional distillation at liquid nitrogentemperatures, retaining only the middle portion of, each distillate. Mass. spectrometric analhsis showedthe purity to be greater than 99.5 percent with the impurity consisting almost entirely of ethane. Nounsaturated hydrocarbons or higher paraffin hydrocarbons were detected, indicating that these substancescould not comprise more than 0.05 percent of the total. The pressures of methane used in the absorptioncell varied from 0.04 to 420 mm Hg.
12.1. ABSORPTION COEFFICIENTS
Figure 30 is a plot of k vs X for methane between 1065 and 1610 A. There appears to be continuousabsorption over the entire range of observation, with some weak bands superimposed in some regionf. Two
66
flat maxima and a possible third were observed. The first two lie at about 1260 A and 1190 1, both having
k-values of about 500 cm-'. The third at about 1075 A, having a k.value of 700 cm-"', is not certain, since
this region is near the transmission limit of the LiF windows. The 1190 A continuum seems to have super-
imposed on it several relatively weak diffuse bands. Above 1450 A there appear eleven bands that are also
diffuse and weak.
Where comparisons can be made, the k-values agree with those of Wilkinson and Johnston (1950) only
to a limited extent. At the lowest wavelength they measured (1370 A), their value is about ten percent
lower, but above 1400 A the agreement grows steadily worse, their value being about an order of magnitude
greater at 1440 A. It is difficult to explain why they reported no k-values below about 1. With the cell
leugth (38 cm) and pressure (40 cm) they used, Io/I - 2.7 for a k-value of 0.05 cm- 1.
12.2. LONG WAVELENGTH LIMIT OF CH, ABSORPTION
The question of the long wavelength limit is not easy to answer. Wilkinson and Johnston (1950) give
1455 A for this value. Since only one point is presented in evidence and no k-values below I are given,
one cannot help but feel that the result is open to question. The work of Duncan and Howe (1934) who
found CH 4 absorption to 1450 X is cited by Wilkinson and Johnston as evidence in support of the 1455 A,value. Duncan used a path length of 200 cm and a maximum C114 pressure of 1 mm Hg. With this layer
thickness and an absorption coefficient of 0.1. h/I - 1.01; if k = 0.5, Io/I = 1.06. Such small Jo/I ratio@
are very difficult to detect photographically and Duncan admits that his long wavelength limit might easily
be in error, because of his low layer thickness, and that the value of 1850 A found by Leifson (1926) and Rose
(1933) independently, might be correct. Wilkinson and johnston state that the oxygen impurity in CH,
must be less than 0.00014 percent and that the continuous absorption up to about 1800 and 1869 A observed
by Leifson and Rose was presumed to be due to oxygen. The k-value of O in this region has been measured
in the present investigation and found to be of the order of I cm-1. Therefore, using Leilson's layer thick-
ness, attributing his absorption to oxygen would have required that his methane contain mostly oxygen
rather than methane. This is deemed unlikely.
The long wavelength limit of absorption obtained in the present investigation is about 1650 A. As
compared to the photographic method, lower ratios of lo/i can be measured by the photoelectric method
since the photomultiplier current is linear with intensity. By careful measurements, k-values are fairly
reliable even when Jo/i is as low as 1.05 and, therefore, in the case of methane, k-values as low as 0.02 cm- 1
were measured with a cell length of 4.7 cm and CH 4 pressure of 420 mm Hg. The high value of the long
wavelength limit obtained by Leifson may be attributed to photochemical decomposition of CH4, since he
placed the absorption cell near the light source. Photochemical products such as ethy!ene and acetylene
absorb strongly at longer wavelengths. In this regard, the present work has shown that many gases have an
absorption in the Schumann region which is comparable to that of 02, and it is not safe to analyse only for O.
12.3. CONTINUA AND BANDS
As noted earlier, methane possesses two and possibly three regions of continuous absorption. One
region has a maximum at about 1260 A. This is in good agreement with the value of 1255.5 A measured by
67
the dispersion techniques of Friberg (1927). Mulliken (1935 b) attributes this absorption to the transition'A1 -+ 'Tt((p) -- 3s). The second continuum having a maximum at 1190 A has several coniparativelIweak bands superimposed on it. It is believed that these bands are real, since they. contribute about 20 cm-to the k-value. This contribution is well outside the limit of error. No attempt has been made to classif%these bands, since their diffuse character makes assignment of wavelength difficult. The approximatewavelengths are 1178, 1190, 1201, 1207, 1218 and 1230 A. The question of these bands being due to im-purities must be considered. The pii-it', of the methane was carefully checked by mass spectrometric anal-ysis and the only impurity found present in excess of 0.05 percent was ethane. Since these bands contributeabout 20 cm-1 to a k-value of about 500 cm- '.the impurity would have to comprise -4 percent of the methaneif its k-value were comparable at this wavelength. Alternatively, it would have to comprise 0.4 percent ifits k-value were 5000 cm-'. The most probable impurities remaining undetected are oxygen, water vaporand carbon dioxide, but since none of these fit the above requirements, their presence may. be ruled out.That these bands may be due to ethane may be ruled out, since the strong absorption of ethane is primarilycontinuous and not banded as shown by Price (1935). In the region from 1462 to 1606 A, eleven absorp-tion bands were noted. Leifson has reported some of these, although Rose has pointed out that they mighthave been due to gaps in the hydrogen spectrum. Although this difficulty may arise photographically, itis unlikely that it will be encountered in photoelectric measurements, since light intensity is measuredlinearly. Similar considerations to those above may be used to eliminate impurities as the cause of thesebands. The wave numbers are 62,260; 62.970; 631530; 64,180; 64,810; 65,360; 66,010; 66,620; 67,200; 67,800and 68,400 cm-. T'he "suer , ;pa ,:)n i:• about 600 cm-u and the intensity seems to decrease as theregion of continuous absorption is approached.
13. AN APPLICATION TO TIlE D LAYER
In this section an attempt is made to apply some of the absorption data to atmospheric physics. Forthis purpose, the D layer seems to be an appropriate subject, since the spectral range covered in the presentinvestigation includes the atmospheric window in the region of Lyman alpha which has often been associ-ated with the D layer.
Several proceses have been suggested in the past for the formation of the D layer (see Introduction)and discussed by man'- investigators. For example, Bates and Seaton (1950) have evaluated the meritsand defects of these theories on the basis of available data. Their paper provides the necessary backgroundfor the following discussion.
The photoionization of atomic sodium by solar ultraviolet in the region from 1800 to 2300 A,Na + hy -- Na+ + e, (5)
has been proposed by Jouaust and Vassy (1941) and by Vassy and Vassy (1942). In this proposal, theyrejected Prestor 's (1940) k-value of 02 at Lyman alpha and considered that solar radiation in the spectralregion from 1100 to 1300 A did not penetrate into the D layer. The data obtained in the present workagree completely with the k-values of all foar gases at Lyman alpha obtained by Preston. Moreover, thek-values of the several windows 7 )2 in the region from 1100 to 1200 A have been found to be about the
68
d J\A t
z1
SII
C')tZ •
0
0.
00D
"" 1 i I I 11050 1100 1150 1200 1250 1300
WAVELENGTH (A)
Fig. 31. Aslorption mce inciono of NO and N0.
same as that of Lyman alpha. Regarding the photoionization rate of pmzess (5), the estimates of Batesand Seaton were accepted in this discussion. Nicolet (1949), however, has considered that this process
produces the normal D layer.
Of the several processes suggested for the formation of the D layer, the following appear to have the
strongest support:
Og + hr (900 to 1000A, spectral region A)--,-O%+ -e (6)
NO + h (1100 to 1300 spectral region B) --* NO+ + e. (7)
Mitra (1951) has reported a detailed calculation to show that the D layer is produced by process (6), whichwas originally proposed by Mitra, Bhar, and Ghosh (1939). On the other hand, Bates and Seaton (1950)have shown that the D layer may very well be produced primarily by process (7) which was suggested earlier
by Nicolet (1945). The lat-er, however, used process (7) to account for the increased ionization in the D
layer during solar flares and not for the normal D layer.
Spectroscopic data seem to show that the D layer cannot be produced chiefly by process (6). Aspointed out by Bates and Seaton, spectrograms by Hopfield (1946) show that a 4-mm layer of air completelyabsorbs radiation in spectral region A (see Eq. (6)). Tanaka (1953), who used a 3-m grazing incidence
spectograph and both the Lyman discharge and the helium continuum as source, confirms Hopfield's result,and estimates from pressures used that the windows in spectral region A are about two orders of magnitude
69
less transparent than those in region B. Price and Collins (1935) have observed an absorption continuum
at 1100 A which extends to shorter wavelengths with increasing intensity. Spectrograms by Hopfield andTanaka also show thi3 continuum.
The absorption intensity of Ot down to 1060 A is shown by the solid curve in Fig. 31 where the ordinateis expressed in cross section (cm'). The continuum mentioned above appears at about 1100 A and the crosssection in the region from 1060 to 1100 1 is about 10-" cm2. On the basis of the data obtained by Hopfieldand Tanaka, the cross section should be stili larger in region A which is utilized in process (6). A reason.able value for air in the spectral region A would be about 10-1 can2 rather than 10-1 - 5 X 10-22 which were
used by Mitra (1951). If the higher value is accepted, process (6) would be placed in the lower E layer
Thus, our data tend to confirm the conclusion of Bates and Seaton (1950).
In contrast, process (7) has the very attractive feature that NO can be ionized by Lyman alpha. Thisline has now been observed as a strong, solar emission line in a rocket spectrogram obtained by the Univer-sity of Colorado (1953) at an altitude range within the D layer.
The concentration of atmospheric nitric oxide is not known. If the tentative estimate made by Bates
and Seaton (1950) from a spectrogram obtained by Durand, Oberly and Tousey (1949) is the correct orderof magnitude, our absorption data for NO seem to indicate that the major part of the production of the D
layer can be attributed to process (7).
The absorption cross section of NO is also shown in Fig. 31 by the dashed curve. It represents a mod.erately strong absorption continuum with a number of weak diffuse bands. The experiment on photoioniza-
tion described earlier showed that a major part of the NO absorption in this spectral region leads to photo.ionization. The data for the ionization cross section are not precise, but they do show that the cross sectionis much higher than was previously estimated. For example, the ionization cross section of NO in theappropriate region for process (7) is about 25 times larger than the estimate used by Bates and Seaton (1950).
The solar spectral intensity in the region from 1100 to 1300 A is not known. However, Watanabe,Purcell and Tousey (1950) have estimated that the mean solar intensity in the spectral region from 1050 to
1240 A corresponded to a black-body temperature of about 5200* K. This value is considerably lower than
the 60000 K value assumed by Bates and Seaton. It is expected that rocket-flown experiments will soon
provide reliable data on the solar spectrum and the distribution of atmospheric NO.
The data supporting the NO theory of the D layer are still insufficient. In spite of the considerable
differences in the estimated and the experimental values of the parsmeters, the net result obtained by Bates
and Seaton appears to be remarkably realistic.
14. ACKNOWLEDGMENTS
The authors express their thanks to Dr. Y. Tanaka for many helpful discussions. His contribution
and that of Mr. F. F. Marmo are acknowledged in Sections 4, 5 and 9. We are also indebted to Corporal
C. Turman and Mr. L. M. Aschenbrand for assistance in assembling the data.
We take this opportunity to express our appreciation to the staff of the Geophysics Research Directorate,
AF Cambridge Research Center, for their interest and encouragement in this work.
70
15. APPENDIX
TABLES OF ABSORPTION COEFFICIENTS
Tables (10 through 14) of k-values of 02, NO, C0 2, N20 and H.0 have been included in the Appendix,since in some cases they may be more convenient than the corresponding intensity curves. The data coveronly those spectral regions in which the k-values are independent of pressure, or nearly so (i.e., the region ofcontinuous absorption and very diffuse bands). The k-values are averages of several values (differentpressures). Altbough the wavelengths are read to the nearest 0.5 or 1 A, they may be in error by 1 or 2 A.
71
Table 10. The k.values for 0, in the region from 1160 to 1750 A.
k x k x k k
1166 0.5 1230 10 1336.5 59.1 1547 21768 1.0 31.5 13 39.5 60 3 51 20969 2.1 32.5 14 43 75 55.5 20472 44 33.5 20 45 93 62.5 19474.5 38 34.5 20 49 155 69.5 175
75 36 35 12.3 51 191 72 17176 24 36 14 55 191 77.5 16577 23 37.5 13 61 220 81 15878 11 38.5 9.3 66 259 85.5 15580.5 5.6 39.5 8.1 69 303 89 144
81.5 11 40 6.3 75 332 91 13782.5 6.1 40.5 4.9 78 342 96 13384 7.1 41.5 870 84 354 1602 12586 3.9 43.5 940 91.5 359 08 11288 2.8 45 95 94 366 13 104
88.5 2.9 47 56 1400.5 370 20.5 9289.5 0.4 49.5 39 05 375 23.5 8990 6.2 52 28 08 376 28.5 8091.5 5.0 54.5 23 10.5 380 33.5 7592.5 10.1 56 19 18 380 36.5 72
93 6.7 57 16 27 376 38.5 6994.5 12.7 59.5 12.S 30 375 44 6395.5 16.5 60.5 10.8 33 377 48 6098 32 62 11.9 36 375 54 54
1200 51 63.5 9.2 40.5 371 58.5 49
01.5 90 64.5 6.5 43.5 372 63 4602 104 66 4.9 50 370 67 4203.5 230 69 3.2 55 368 71 3905 500 71 1.8 57.5 365 77.5 3506 480 74 2.5 60 361 82 32
06.5 460 77 4.1 6Q 355 87 3006.5 330 79.5 6.7 67.5 355 89 26.509 130 83.5 9.8 73.5 350 97 24.010 87 87 12.7 79 338 1702 21.811 24 90.5 14.6 86 324 05 20.3
13 17 93 15.7 89 318 12 18.213.5 11 96.5 14.8 91.5 319 17 16.415.6 0.3 99 14.0 95 307 22 15.017.5 0.8 1302 11.9 99 304 27 13.618.5 2.5 06 9.6 1504 294 32 11,9
20 5.2 09 13.9 10.5 285 37 10.621 5.9 12.5 19.4 17 274 42 9.522 11 17 29.6 22.5 266 47 8.323.5 7.0 21.5 42.9 32 249 49 7.225 13 25 55.0 37.5 233 51 6.5
28.5 11 29 62.1 41.5 22729 12 33.5 61.6 44.5 219
ri
72
Table 11. The k.values for NO in tme region from 1060 to 1360A.
k k k k
1069 134 1140 113 1222 60 1300 S8
71 1.€6 41 9S 24 57 02 5375 176 42 96 26 55 03 4878 214 43 101 31 52 04 4580 233 46 91 35 58 05 51
83 176 50 84 37 56 08 5385 215 52 30 40 54 10 5788 146 55 75 43 53 12 5891 129 57 79 45 53 14 5493 126 58 71 48 55 16 48
95 129 62 70 50 67 18 4799 114 66 69 55 52 20 52
1101 120 67 68 57 54 22 705 129 70 67 S8 57 25 8707 173 72 71 61 59 27 94
08 170 73 72 63 54 29 8010 151 77 68 65 SI 31 5613 116 S0 65 67 53 32 4515 105 82 63 70 58 34 5316 105 84 59 72 58 35 83
19 96 86 66 75 63 37 9021 108 90 64 78 51 38 8=23 93 92 66 79 AS 39 6525 96 93 60 81 47 41 5626 110 97 58 83 58 43 42
29 101 99 59 84 62 45 4632 9 1202 62 87 77 46 5333 82 04 62 8a 76 48 5934 82 07 57 91 67 50 5635 92 10 57 92 49 52 57
36 90 12 59 93 50 54 4237 103 14 58 94 55 56 4038 106 15.6 68 96 65 5L 3239 108 19 61 96 63 61 85
73
-~ ~ ~ o i C 'Iý
6i. . . . c Li Ii "i Li~ Ii LiIi Liki
An m -- te meI
11U CU1 CC ,. ~ U , In , 00 1, U,
N 40f (i O
-lci i --ci ' - --- --- - -- - -
U ~ ~ 4i-c -f t- c IAO 0c I- r ' 4U,'-3 C~ ~ 0
-A N N - - - - 4~c- -~ Inc IM '.6 'Z t- It
"I On .. t- r- w inr- i Ný') ; o
InI On N cot t-C 00 1 0' 00 f-i C- I
ill- 1- N1 Ni C1 CIO
P,,U o'U It) It) mf sin UU ;7, U, ,U . 0, m, 4
-1 go0ci 'i' -0 - nc
foi ' 'D I m O 10 1 - -- - -. l,- - c - ci 'iCiý'
'4 i In .1 oO 0'i IO ii ~ .c- ' ~ ~ 0 0 ~ O Cl~c ix I -g . 4- ta s ~ 3 s'~3 0 0010 - -R - -m 8 c s i s ic i ,U
74
I"f"944"in ~to4 t 94C4~ 4.4 ~ 040 00 0 000 0 0 000C
- - -4 Z; p St-
; 4 146
.. V7 .5 .5 . .5. .5U . . . . .
PI . 4. 4-1 11 .41 -1 . ý ý - w 4 en0-n4" i n n m o
Z3C 0 S V0~ t-r -c 4t- t -r -tC.1-0 z E-M ix A4 t-A-0 00000t r o m L 0 r- a*O
In 6C.40 C!CCCN- 4-
94 -4 d" e4 1- 4 0 0 ;C4ci6 -4 0 4*CP- P .41"1
0.I4O Li 1ý li LiU i iL i i6 l i
L4- 44 C4 - -4
SaS VS sk .5 ig s5 8 14" 4 1"0 5 311,AA: 14 3 1#, 3
944~ NC ellIS on It a5-~e a 0 4 '0 0 r- ens'O%0%Os O%0 -. - -4'4' -V -%CC~ e q1'~%
rl 000 000 000c C i0 i i L iUC4 -N v 64 a %4-, 3 ~ -
-4 t-C4. 1c
0000 00 00 04 0 0 00
ea CdC4d A L 0 c00C4 4 C4 4
.55. U5 .5.5 Li Li LS...5iS
Ina-4M L ',0 '
75
P 9! it n t fn4 fr-ti@u0- C4C 44 -1 C-- C-4 @4 04
"t~- - -- ---. ---- --i Ili -- -i - -i
-A -e MA Mn Mn~i Onn in 00 4. g " 1t- C
iti d4@4 a' is~IV to-, S!; g8 s8i 1: n n4'0C ZR ;s sts
---- CC -- 2-228 s -- s a- !v ;
,c .4 4f Ita' -0 - - R P P - - @t - -@4 R4 a4 A4 @ 0@ 0 -r- -M -s - -0 - 6- -
CC L
- @4
4 1 -0ZW toW %P R~ L. I C 4wl @p .0
ininLi l in Ui in Li ni 1n Li n i U ! iniinLinU l nin L,
x - -C-rt-Pi 0 2 18%5 9ItazclA" - 3A"
76
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GEOPHYSICAL RESEARCH PAPERS
No. I. Iorzopic and Non-leotropic Turhulence in the Atmospheric Surface lAIyar, liei.ix Lttau, Geo-physical Rletearch Directorate, December 1949.
No. 2. (Classified)
No. 3. Diffraction Effects in the Propagation of Compressional Waves in th,. Atmosphere, Norman A.ilaskell, Geophysical Research )irectorate, March 1950.
No. 4. Evaluation of Results of Joint Air Force - Weather Bureau Cloud Seedi;x ; Trials Coudatcted DaringWinter and Spring 1949, Charles E. Anderson, Geophysical Research Di•ectorste, May 1950.
No. 5. Inveatigai:ion of Stratosphere Winds and Temperatures From Acoussical Propagation Studies,Albert P. Crary, Geophysical Research Directorate, June 1950.
No. 6. Air-Coupled Flexural Wavews ia Floacung Ice, I'. Press, M. Ewing, A. P. Crary, S. Katu', .,'. Oliver,Geophysical Research Directorate., November 1950.
No. 7. Proceediags Of the Con0eCence Dn Ionospheric Ree,-r,-N (June 1949), edited by Bradford B.Underhill and Relaph J., Donaldson, Jr., Geophysical Research Dire.tors,.:, De-embr 19.,5 0.
No. 8. Proceedings oil' the Colloquium on Mesospheric Physics, edited by N. C. Gerson, GeophysicaResearch Diviýjioo, July 1951.
No. 9. The Dispersion of Surface Waves ou Multi-Layered Media, Norman A. Haskell, Geophysics Re-search [Divisioitk, Augaat 1951.
No. 10. The Meemiuremout of Stratospheric Density Distribution With the Searchlight Technique, L.Elterman, Geop.yvice. Research Division, December 1951.
No. 11. Proceedings of tie Conference on Ionospheric Physics (July 1950) Part A, edited by N. C. Gersonand Ralph J. Doaaldson, Jr., Geophysics Research Division, April 1952.
No. 12. Proceedings of Lýhe Conference on loroapheric Physics (July 1950) Part B, edited by LudwigKatz and N. C. Gerson, Geophysica Reasearc- Division, April 1952.
No. 13. Proceedings of the Colloquium on Microwave Meteorology, Aerosols and Cloud Physics, editedby Ralph J. Donaldson, Jr., Geophysics Research Division, May 1952.
No. 14. Atmospheric Flow Patterns and Their %cpret~entation by Spherical-Surface Harmonica, B. Haurwitzand Richari4 A. Craig,. Geophysics Research Division, July 1952.
No. is. Back-Scattering o:& Electromagnetic Waves From Spheres and Spherical Shells, A. L. Ades, Geo-physics Research Dirision, July 1952.
No. 16. Notes on the Thlo-iory of Large-Scale Disturbances in Atmospheric Flow With Applications toNumerical Weather Prediction, Philip Duncan Thompson, Major, U. S. Air Force, GeophysicsResearch Division, July 1952
No. 17. The Observed Meai Field of Motion of the Atmosphere, Yale Mintz and Gordon Dean, GeophysicsResearch Directorate, August 1952.
No. 18. The Distribution of Radiational Temperature Change in the Northern Hemisphere During March,Julius Loudon, Geophysics Research Directorate, December 19152.
No. 19. International Sympsainum on Atmospheric Turbulence in the Boundary Layer, MassachusettsInstitute of Technology, 4-8 June 1951, edited by E. W. Hewson, Geophysics Research Directo-rate, December 1952.
No. 20. On the Phenomenoc 1,1 the Colored Sun, Especially the "Blue" Sun of September 1950, RudolfPeundorf, Geophysii•a Research Directorate, April 1953.